equilibrium will be restored. The cylinder в therefore, loses, after its immersion, a part of its weight equal to the weight of the water poured into the cylinder A. Thus, the principle of Archimedes is proved, since the internal capacity of the latter cylinder is exactly equal to the volume of the cylinder B. Determination of the volume of a Body. The principle of Archimedes supplies us with the means of finding with exactness the volume of a body of the most irregular form, when it is not soluble in water. For this purpose, we hang the body by a fine thread to the hook of the scale of a hydrostatic balance, and weigh it first in air of a given temperature; we then weigh it in distilled water at the same temperature; and the loss of weight which it experiences in the latter case, is the weight of the water displaced. From the weight of this water we ascertain its volume, and consequently that of the body immersed, which is evidently the same. For example, suppose the loss of weight in a body to be 2lbs. Avoirdupois, the temperature of the water and air being 62° Fahrenheit; it is plain, since an Imperial gallon of water, at this temperature, weighs 10 pounds, and that its volume is 277-274 cubic inches, that the volume of the body in question must be one-fourth of that of an Imperial gallon of water, or 69.318 cubic inches. Equilibrium of immersed and floating Bodies. - From the theoretic considerations which led us to the principle of Archimedes, we see that, if a body be immersed in a liquid of the same density as itself, the upward pressure which tends to raise the body is equal to its weight; the body will therefore, remain suspended in the liquid wherever it is placed. If the body has a greater density than the liquid, it will sink; because the pressure of its weight is greater than the upward pressure of the liquid. If the body has a less density than the liquid, the upward pressure of the latter will predominate over that of its weight, and the body will float at the surface, having displaced only a quantity of the liquid of the same volume as itself. Wax, wood, cork, and all other bodies lighter than water, therefore, float on its surface. The laws of stable equilibrium relating to bodies immersed or floating in a liquid, are: 1st, that they displace a quantity of the liquid whose weight is equal to theirs; 2nd, that their centre of gravity must be below the centre of pressure and in the same vertical line. In theoretical mechanics, however, it is proved that stable equilibrium may take place although the centre of pressure be found below the centre of gravity, provided that a certain point called the Metacentre, be situated above the centre of gravity. The determination of this point belongs to geometry; and its knowledge is of the greatest importance in the stowage of vessels, for it is upon the relative position of the centre of gravity, and the metacentre, that their stability depends. According to the principle of Archimedes, bodies float with greater facility upon the surfaces of liquids in proportion to their density relatively to that of the floating bodies. For example, an egg sinks in common pump or river water, because it weighs heavier than an equal volume of water; but it swims in water saturated with salt. A piece of oak floats on water; but it sinks in oil. A mass of iron floats on mercury; but it instantly sinks in water and most other liquids. In floating bodies, the volume of the part immersed in the liquid, is in the inverse ratio of the density of the liquid, and in the direct ratio of that of the floating body. The different effects of floatation, suspension, and sinking in a liquid, are ingeniously illustrated by the following apparatus, fig. 36, which consists of a tall glass jar, or similar vessel, nearly filled with water, and closely covered at the top with a piece of bladder or other air-tight membrane. In the vessel is placed a small figure made of glass, metal or enamel, having a hollow ball of glass at the top containing air and water, and floating at the surface of the water in the vessel. In this ball, at the lower part, there is a small aperture through which water is made to flow inwardly or outwardly, according as the air within it is more or less compressed. The quantity of water previously introduced into the ball is such, that the figure requires only a very slight additional weight to make it sink to the bottom of the vessel. If then we press lightly with the thumb, on the air-tight membrane, the air immediately in the ball expands, the additional water is forced out of the ball, the figure becomes lighter, and rises to the surface to float as before. Thus, by varying the pressure, the figure can be made to remain at the top, in the middle, or at the bottom of the vessel, at the pleasure of the experimenter. the back-bone with a thin membraneous vessel full of air, A great many kinds of fish are furnished interiorly under called the swim. These fishes, by compressing or expanding this vessel by a muscular effort, cause its volume to vary, and produce effects similar to that which we have described in the preceding experiment; so that by this means they can rise or sink, or remain in the middle of the water, according as instinct directs their motions. Swimming.—The human body is lighter than a quantity of fresh water equal to it in volume. Hence, it will naturally float on fresh water, such as rivers, ponds, &c.; and still more on salt water, such as the sea, the latter being a heavier liquid. The difficulty of swimming, therefore, arises less from the inability of the swimmer to keep near the surface of the water, than from his inability to keep his head above water in order to have free respiration; for his head, being heavier than the other members of his body, has a tendency to sink. Hence the necessity of human beings learning to swim, and of cultiOn the contrary, the heads of quadrupeds rating it as an art. being lighter than the other members of their body, they can remain for a considerable time in the water without sinking, and can swim naturally without effort. SPECIFIC GRAVITY. Specific Weight or Gravity.—We have seen, in Lesson IV. p. 46, that the specific weight or gravity of a body, whether solid or liquid, is a number which expresses the ratio of the relative of distilled water at the maximum density. In order, therefore, weight of a body of a given volume, to that of an equal volume to determine the specific weight of a body, we must find its relative weight and that of an equal volume of water, at the when the quotient will be the specific weight required, that of same temperature, and divide the former weight by the latter, water being taken for unity. Various methods are employed in the determination of the specific weights of solids and liquids; the most useful of these methods will be explained in the following paragraphs. Specific Weight of Solids.-In order to determine the specific weight of a solid, by means of the hydrostatic balance, fig. 35, we weigh it first in air, and then suspending it by the hook of the balance we weigh it in water; the loss of weight which it experiences in the latter case is, according to the principle of Archimedes, the weight of a volume of water equal to that The Areometer of Nicholson.-The term areometer (from Gr. araios, thin, and metron, measure) literally signifies raritymeasure, and is applied to a floating apparatus employed in determining the specific weights of solids. The areometer of Nicholson is composed of a hollow cylinder в made of tin, fig. 37, to which is applied a cone c, filled with lead; the use Fig. 37. of this cone being to ballast the apparatus in a vessel of water, in such a manner as that its centre of gravity shall be placed below the centre of pressure, the condition, as before remarked, necessary for stable equilibrium. This apparatus is furnished at the upper part with a stem and a scale-pan A, for the reception of the weights and the bodies whose specific weights are to be determined. On the stem a mark is placed, called the water-mark or point of water-level, which is used to determine when the apparatus sinks to the same level in the vessel of water, when loaded with a weight or with a body. In using this instrument, we first ascertain the weight which it is necessary to put into the scale A, in order to make the areometer sink to the water-mark in the vessel; for, when the scale is empty, the instrument floats above the level of the water. Now, supposing that this weight is 2,000 grains, and we wish to find the specific gravity or weight of any substance, say sulphur; we remove this weight; we take a piece of sulphur of less weight than 2,000 grains, and we place it in the scale A; we then add as many grains as are necessary to make the areometer sink to the water-mark. Supposing that these additional grains are 880, it is plain that the weight of the piece of sulphur in this scale is 1,120 grains. Having thus determined the weight of the sulphur in air, we must now find the weight of an equal volume of water. For this purpose, we lift the areometer out of the vessel of water, and we transfer the piece of sulphur from the scale A to the cone c, as shown in the figure. The total weight of the apparatus is not changed by this change of position in the place of the sulphur, and yet on its re-immersion in the vessel of water, it no longer sinks to the water-mark. This arises from the fact of the sulphur, when immersed, losing actually a part of its weight equal to that of the water which it displaces. If we now put into the upper plate A, weights sufficient to bring the instrument down to the water-mark, say, in this instance, 551 grains, this number will represent the weight of the volume of water displaced; that is, of the volume of water equal to that of the sulphur. We have, therefore, only to divide 1,120 grains, the weight of the sulphur in air, by 551 grains, its weight lost in water, and we have for the quotient 2.03, the specific weight or specific gravity of sulphur. If the substance whose specific weight is required is lighter than water, it will have a tendency to float, and will not remain on the base of the cone c. In this case, we employ a small iron-wire grating to cover the body and prevent its rising to the surface of the water in the vessel. This being done, the process of determining its specific weight is then conducted in the same manner as in the preceding example. LESSONS IN GEOLOGY.—No. XLVI. BY THOS, W. JENKYN, D.D., F.R.G.S., F.G.S., &c. CHAPTER IV. ON THE EFFECTS OF ORGANIC AGENTS ON THE EARTH'S CRUST. SECTION IV. THE RESULTS OF THE AGENCY OF MAN. THOUGH man comes into the world as a creature subject to the laws of nature, it is yet evident that he is endowed with an agency that can re-act upon nature, so far as not only to transform her aspects, but even to prescribe new laws to her operations. Man has not been so long upon the face of the earth as plants and animals have been, nor has his race been so extensively distributed as the flora and the fauna of the globe have been, and therefore he has had neither the time nor the space which they have had, for exerting deep and signal influence on the earth's crust. Yet, wherever mankind have established themselves and promoted civilization, there they have produced great geological changes in the surface of the globe. You have often heard the line quoted "God made the country, and man made the town." There is much poetical truth, and some physical truth, expressed in this verse: but it is not "all truth, and nothing but the truth." Geology robs it, not of the beautiful poetry which it breathes, but of the physical fact which it asserts. Man has not only made the town, but his agency, and especially his advancement in civilization, has made the country much what it is. You will, very likely, change your mind about the fact on which this poetry is founded, if you will attend to this lesson. Where man is found in a savage stare, employed merely in hunting or fishing, his induence in changing the aspects of nature is very slight. In this state, the population is comparatively scanty, and its activity is very limited. He may extirpate a few animals for food, and he may destroy a few forests for fuel, but the change which he produces on the earth's surface is very trivial. When man becomes a shepherd and herdsman, and leads a nomadic life, his influence on the earth becomes more extensive and permanent. Particular animals are, by his agency. appropriated, tamed and domesticated. These animals increase in number and multiply. The very process of taming them produces great and decided changes in their habits and aspects. until they appear as if they were altered and become new animals on the earth's surface. In the mean time, the wild animals that are destructive to the tamed breed, are attacked and eventually exterminated. To find pasture for the multiplied domesticated animals, heaths, prairies, and forests are destroyed by fire; and the ashes which result from the conflagration give rise to a more luxuriant vegetation. It is when man becomes a tiller of the ground, an agricul turist, that he exerts the greatest influence in producing geological changes. He now puts forth his energy to root out immense forests, in order to form arable land; or, from his knowledge of agricultural chemistry, he may burn down the forests, that the ashes may manure the soil and increase the amount of the crop. It is well known to travellers and readers that this process of burning large forests, for agricultural purposes, is carried on upon a gigantic scale in North America, in Brazil, in Java, and in many tropical countries, where vast forests have been completely extirpated. As man advances to be an agriculturist, he comes to apply his agency to all manner of soils on the earth's surface. He finds land that is too wet either to produce pasture or to grow corn. He then proceeds to drain the marsh and the moor. The water is drawn away towards rills and streamlets. The beds of brooks and rivers become narrowed, another direction is given to their course, and a new power given to their abrading action. Man can raise effectual barriers against the pro ་ gress even of the sea itself. Man in a civilized state pro- | undesigned contributions of his auxiliaries, has done much to tects the coasts of his country, by raising dikes against the change the face of nature. encroachments of the ocean. He dams in many bays of the sea by artificial embankments, and in this manner abridges the dominion of the sea, and changes the bottom of the ocean, first into pasture land, and then into arable soil. Look to Holland. Man has rescued that country, and is still continuing to rescue it from the ocean. At this very day, man is altering it, and enlarging it by draining immense lakes, and by dredging fresh soil from the bottom of rivers and seas. up It is not likely that the human race, living amid the geological changes which its civilization produces on the surface of the earth, will be able to form an adequate conception either of their physical importance, or of their scientific value. If you imagine that the continents of our globe were once more, as they have been frequently, before, submerged under the waves of the ocean, and that the geologist of some future millennium would be investigating these very complicated phenomena,then, to him, the particulars recorded in the geological works of the present age would be of incalculable value. They would give him new light in his inquiries and new power in his proofs, as he descanted upon the fossil flora and fossil fauna of the rocks which were deposited in, what would then be called, the human epoch. The agency of man, by means of agriculture and by the progress of civilization, exerted upon plants and animals, is developed in three ways: first, by the removal and extinction of one class: secondly, by the introduction and extension of a second and thirdly, by the modification of others. In every country where man settles, some classes of plants belonging to the district are allowed to continue to clothe the soil, and others are displaced and exterminated. In his migrations, man carries with him foreign plants which he conveys from other and distant regions, and sows in his adopted land. This we find to be the case in Europe. All of our corn plants, most of our fruit trees and kitchen vegetables, have been derived from Asia. The potato and tobacco were brought from America. Cotton has been conveyed from India to North America and Brazil. Coffee has been transplanted from Abyssinia and Arabia to Java, to the West Indies, and to South America. What man has done for plants, he has accomplished for animals. When Europeans landed in America, they found the New World to be quite destitute of every one of our domestic animals. Innumerable species of them are now found in all parts of North and South America. This is also the case with South Africa, Australia, Van Diemen's Land, New Zealand, and the islands of the Pacific. You must not allow this fact to pass unobserved, for in consequence of this transportation of new animals, great revolutions have taken place both in the physical aspects of the country, and in the activity of human life on the globe. As man has on the one hand extended the animal kingdom, he has, on the other, exterminated or expelled many entire races of animals. This is the case with the elk and the beaver in Northern Europe, the furred animals of North America, the hippopotamus and crocodile in Egypt, the lion in Greece, and the wild boar and wolf in Britain, One of the most remarkable facts in the results of man's reaction upon nature is, that, by studying and obeying her laws, he has compelled her to bring forth new creations which did not previously exist-creations which are daily increasing and multiplying. This refers to the modified forms of beings, to the rich varieties and to the diversified races of plants and animals which have been produced by man's skill in combinations. The infinite number in the races of dogs would never have existed unless man had studied and mastered the wolf and the jackal. The same endless variety of races might be instanced in the horse and the ox. The case is precisely the same with plants. If nature had not been acted upon by the genius of man, we, instead of the 1,400 or 1,500 differen kinds of apples which now adorn our orchards, would have had nothing but the wild crab; and instead of the innumerable varieties of roses which now fill the air with their fragance, we would have had only the wild rose. This power of man over. vegetative nature is singularly illustrated in the history of the dahlia. At first the dahlia was a simple flower; now, this one plant can boast of fifteen hundred double varieties. One circumstance that gives great importance to the agency of man on the earth's surface, is the fact that it exerts an influence on the formations of climates. The removal of extensive forests, especially in the mountainous districts of warm countries, produces a great and lasting change in the condition of the humidity of the atmosphere. Under the shade of forests, the earth and the air always become cooled. This cooling causes a condensation of vapours in the atmosphere. The condensed vapours fall on the earth as dew or rain, which produce springs in the soil. These springs combine rills, streamlets, brooks and rivers. The felling of forests, therefore, by giving free play to the winds, affect the watershed of a country. The drainage of marshes, the drying up of lakes, and the deepening of river beds, diminish evaporation, and consequently lessen the moisture of the atmosphere. As an illustration of the influence of human agency upon climates, it may be stated that scientific men consider the climate of Europe to be much warmer now than it was in the days of the Romans, and that this improvement is due to the clearing away of the ancient forests. Though this has been disputed by the late M. ARAGO, there can be no doubt that, by the extermination of the forests, the climate of the different states of North America have been very much modified. There are innumerable instances in which the plants, that were first introduced by human settlers, have, without his will, and sometimes even against his will, multiplied and diffused themselves so widely, as even to displace the original vegetation of the country. For instance, in St. Helena, the original flora of the island has been almost driven out by the foreign plants which were brought there since it became inhabited by Europeans. Also, in the pampas of South America, in New England, in Australia, in South Africa, the European species of plants, which have been brought thither by man, exceed in number all the other species which have been wafted from any other region and by any other agency. Sine of the facts connected with the transmission of plants are full of interest. It is well known that in modern times, armies have been known to carry, in all directions, grains and vegetables from one extremity of the earth to another. This fact throws light upon the diffusion of vegetation, and shows that in more ancient times, the conquests of Alexander, the distant expeditions of the Romans, and the marches of the Crusaders, have transported many plants from one region to another. Where man introduces corn, he introduces corres- The mining operations of man have had some small share in ponding weeds also. In our own corn-fields, both the grain producing geological changes in the superficial crust of the and the weeds that grow with it are from Asia. In the south earth. These mines are to be distinguished from the quarries of France, where the farmers are in the habit of sowing Bar-which he has opened in the face of mountains, whether for bary wheat, there also the weeds of Algiers and Tunis continue dislodging any vein of ore that has its outcrop at the surface, to grow. Even the wools and the cottons which are brought or for removing freestone and slate for building purposes. to France from the East, have borne with them the grains of By mining, he digs a perpendicular shaft many fathoms deep exotic plants which have naturalized themselves near Mont- into the bowels of the earth, or opens a horizontal gallery, pellier. Outside cne of the gates of that town, is a meadow called a level, which enters into the side of a mountain and which is set apart for drying foreign wool after it has been may penetrate the rock for many furlongs and miles. In this washed. Now, in that drying-ground, there is scarcely a manner immense seams of coal and beds of salt are extracted in which some foreign plants are not found growing and from their place amid the deposits of the globe, and extensive year naturalizing themselves in the soil. This interesting fact veins of the ores of iron, lead and copper and other metals are illustrates how seeds are bore from one region to another, in removed from beneath the earth's surface. These mining the woolly or hairy coats of wild animals. It is hence obvious operations tend to produce irregularities in the earth's sur that man, partly by direct and positive agency, or by the face. He brings upwards from the depths below an enormous quantity of clays and stones, which he piles in heaps upon the soil. By the wide excavations and the empty spaces which his adits have left in the rocks beneath, certain extents of the earth's surface fall in and form hollows. Compared with the magnitude of the globe, and the depth of the semidiameter of the earth, all these minings are mere burrowings and scratchings: still they have not been without their influence in the geological changes of the earth's crust. The last instance to be mentioned in this lesson, of man's agency in affecting the crust of the earth, is the contribution which he makes of particular fossil remains to the various rocks that are now being deposited both on land and at the bottom of the sea. In the vast hollows which man's mining operations have produced in the rocks, an immense quantity of timber is introduced, to prop up the top of other rocks and revent them from falling in. It has been calculated that, in the Cornish mines alone, it would require one hundred and forty square miles of Norwegian forest, to afford the due supply of timber for the works. Many of the soils and rocks that are now forming, abound with the various productions of man's art, the remains of buried towns, the wrecks of ships containing human bones, the ruins of machinery, pottery, and a great variety of coins, trinkets, &c. The skeletons of the human frame are also daily contributing a large amount of fossils to the deposited rocks, in burying grounds or in battle fields, and in the depths of the ocean. From the facts and statements contained in this article, you will see that man has contributed something to make the country as well as to make the town. means by which the separation could be effected, but they are the most evident, and the means immediately deducible from the evidence before us. As regards the separation of arsenic by means of hydrogen, the operation may be said to apply to all cases whatever; its value will therefore be easily recognised, arsenic being a very important metal, and frequently coming under the notice of a chemist in cases of poisoning by it. I shall now pass on to the performance of other experiments having reference to its separation from matters which contain it. I shall, firstly, rely on the two prime agents of analysis (hydrogen and sulphuretted hydrogen), and shall then proceed to mention some processes which may be required as supplementary. Experiment.-Mix together a little arsenious acid in the form of liquor arsenicalis, with about a wine-glassful of milk, and proceed to extract the arsenic. How would you set about it? Doubtless, our two agents will occur to you. You will either determine to throw down the arsenic at once from the milk by means of a current of hydrosulphuric acid, or you will determine to pour the mixture into the proper apparatus, and with the proper materials for generating arseniuretted hydrogen gas. All very well in theory; but in practice it will not do. You will soon find, on trial, that sulphuretted hydrogen does not well act in milk,-hence that plan is ineligible;-you will as soon discover that the milk causes such a bubbling in the bottle for generating arseniuretted hydrogen, that instead of more gas escaping, as it ought, the liquid comes out in a jet. You must therefore commence by getting rid of a certain portion at least of the animal particles of the milk. A definite object is thus presented to us. Now most people know that the portion of milk termed caseine is coagulated by the operation of acids;-acetic acid (distilled vinegar will be heated, coagulation will ensue; filter through gauze, and wash the coagulum-you obtain most of the arsenic. Another portion of milk is rendered insoluble by high drying; hence if the fluid which has run through the gauze be evaporated, not only to perfect dryness, but until a small slip of deal wood a, The series of lessons to which you have hitherto attended, have been arranged in such a manner as to make you acquainted with the various agencies which, under the super-do) ;—if, then, a portion of acetic acid be added, and the milk intendence of the Supreme and Benevolent Architect of the world, have been employed in constructing the crust of the earth. You have seen that fire, water, atmospheric agents, winds, plants and living organisms have co-operated in the production of the shell of the globe. Our next series of lessons will be on the classification of the rocks that compose the earth's crust. LESSONS IN CHEMISTRY.—No. IX. In a science of such vast extent and seeming complexity as chemistry is, great care should be taken to present each subject under its simplest possible aspect, and never, on any occasion, to lose the thread which is to conduct us through our labyrinth. Now, I am aware that objection may be taken to these lessons, of the following kind. It may be argued that I have omitted many important tests; that I have not taken cognisance of numerous oxides; not even to the extent of mentioning their name that I, on some occasions, have not employed the term or designation which chemists have proved to be most correct; as, for example, I have simply used the term sulphuret of arsenic, instead of the more precise term sesquisulphuret, indicative of the amount of sulphur which this particular sulphuret contains. I will not apologise for this plan of treating the subject, inasmuch as I know it to be the best plan, but will simply assure the student that the propriety of all these omissions has been well observed in my mind. Again, I have, in a previous lecture, asserted roundly that the salt sulphuret of zinc is a compound of sulphuric acid and -oxide of zinc,-and that generally metallic salts are compounds of acids and oxides; yet I am well aware that the expression is by no means universally correct. In a word, it is my intention to avoid theory altogether in these lessonsnot that I undervalue theory, but I consider that it is treated of with the greatest propriety alone. Resuming the consideration of arsenic, you will see that two distinct properties have been brought into requisition :two distinct lines of action for effecting the separation of arsenic. If combined with zinc or manganese, the only two metals which, in addition to itself, have hitherto come under our notice, it may be separated, as we have seen, by the agency of hydrogen: this is one power. If combined with manganese alone, it may be separated, as is here seen, by hydrosulphuric acid, which, as we have proved, throws down arsenic, but not manganese. You are not to assume that these are the only pushed quite down to the bottom of the dish, and there caused to remain during the operation, become slightly browned, you will find that the residue being allowed to cool, and water added, the resulting solution will be still further disembarassed from animal particles. The fluid will now, in the case of milk, be sufficiently pure to admit of working satisfactorily, or by the arseniuretted hydrogen plan, and will even afford a satisfactory result with hydrosulphuric acid. The process of coagulation by an acid only applies to such an organic matter as milk; but the process or high drying is applicable universally to all kinds of organic mixtures. In order to ensure the total separation of organic matter, other measures should be adopted; but those described should be firstly tried, and are generally efficient. Further Tests for Arsenic.-Prepare a clear, weak solution of white arsenic (chemical term arsenious acid) in water and potash, either by the process already described, or by the more direct process of boiling a little white arsenic with potash solution. When prepared, acidify a portion of this solution, and apply the following tests: (1.) Ammoniacal Nitrate of Silver.-Take a bit of lunar caustic (nitrate of silver), or the same salt in a crystalline state, if you can obtain it, and add to it a tablespoonful of water: a solution will thus result, which should be kept in a glass-stoppered bottle;-very frequently, hereafter, we shall require this solution, nitrate of silver, as a test. (2.) Add a few drops of this solution to a small portion of arsenical solution, placed in a wine-glass, or some other convenient vessel. If the solution have been duly acidulated with acetic acid, as directed, no precipitate will ensue. Add now very minute quantities of liquor ammonia (hartshorn) by means of a glass rod ;-a yellow precipitate will now immediately form; hence a combination of nitrate of silver and ammonia, used as directed, becomes a test of the presence of arsenic. (3.) Add now more ammonia, and you will observe the precipitate just formed to dissolve entirely ;--thus proving the necessity of attending to the conditions of success. At a time when the indications of this test were more valuable than they now are, it was usual to keep ready-made in the laboratory, a mixture of nitrate of silver and ammonia in due proportion,-under the name of Ammonia nitrate of silver. It may be made by adding to a solution of nitrate of silver sufficient ammonia to precipitate all the oxide of silver, and dissolve it nearly, though not quite. N.B.-Particularly remark that no precipitation occurs in such a solution as above described, until a certain quantity of ammonia has been added. will be burned away; and if the animal or vegetable compound should have chanced to contain arsenic, the latter would have been simultaneously converted into arsenic acid. Experiment (5.)-Mix a little arsenious acid, in any state of mixture, with some milk, porter, cabbage, or, in short, any animal or vegetable substance :-add nitre to the mixture; evaporate, dry, and ignite in the best way you can, The operation should be conducted in a platinum crucible; it may be effected to a sufficient extent for demonstration on a slip of thin glass. Redissolve, filter,-precipitate by hydrosulphuric acid, and reduce the precipitate to the condition of metallic arsenic, as before described. The process of extracting arsenic Conversion of Arsenious into Arsenic Acid.-Hitherto I have from animal and vegetable mixtures, after previous conversion designedly employed the term arsenic in a somewhat loose into arsenic acid, has been frequently had recourse to in judicial sense sometimes meaning by it metallic arsenic, sometimes inquiries. The late Professor Orfila first taught medicowhite arsenic, or arsenious acid. Inasmuch as metallic arsenic legal inquirers that, in many cases, it was not enough to seek does not dissolve as such--and arsenious acid is that alone for arsenic in the contents of a stomach, inasmuch as the which has entered into all our solutions hitherto,-no great poisonous agent might have by chance become absorbed into precision of language has been called for. We shall presently, the liver or other tissues. We shall presently, the liver or other tissues. This being the case, the process of however, discover that arsenious acid is not the only comincineration with nitre becomes especially advantageous: pound of the metal which can enter into solution :-there occasionally, however, the same ultimate result may be acbeing also an arsenic acid, the study of which substance in-complished by the use of even better substances than nitre. volves some important points. Such variations of the process, however, need not be discussed just at present. What is the difference between arsenious and arsenic? or, more generally speaking, what is the distinction intended to be conveyed by appending ous and e respectively to any prefix? The explanation is simply this. The greater number of acids contain oxygen, and one substance frequently combines with oxygen in two or more proportions to form two different acids. Provided the number of combinations be no more than two, the acid containing the smallest quantity of oxygen is designated by the termination ous, the other by the termination ic. The former makes salts called "ites," the latter those called "ates." Thus, in the present case, we have arsenious and arsenic acids; arsenite and arseniate of potash. Composition of arsenious and arsenic acids :— Experiment (1.)-By fracturing a Florence flask almost at random, curved stripes and spicule will be obtained, which are very useful in many chemical operations. Place upon a thin, curved strip of this kind, a little of a mixture of arsenious acid and nitre (saltpetre), otherwise called nitrate of potash, and fuse the mixture by the heat of a spirit-lamp flame. Maintain the mixture in fusion during two or three minutes, then remove the source of heat, and allow the mixture, glass and all, to cool. When it has become quite cold, break the glass with its fused coat into little pieces, if necessary; put them into a test-tube, pour water into the tube, and apply hea. By following these directions, a solution will be obtained, in which the arsenic exists-but not as arsenious acid. It will now have acquired oxygen from the nitre, and become arsenic acid. Detection of Arsenic by Sulphuret of Copper and Ammonia.all mention of this test,-which is one of considerable deliIt would not be proper, under the head of arsenic, to omit cacy,-although far inferior to sulphuretted hydrogen, and the process of extraction by hydrogen. This Experiment (1.)—Into a portion of liquid containing arsenious arsenic. little washing soda, and heating in a tube: charcoal here is I may here mention, in connexion with the mixture of Experiment (2.)—Test a little of this solution with nitrate of Experiment (3.)-Pass a current of hydrosulphuric acid through another portion of the same solution, and observe that a ye low precipitate falls as it would have done in a solution of arsenious acid. The precipitate, however, although similar in colour and behaviour, is different in the mutual relation of its components. cover the carbonate of soda, but charcoal and carbonate of potash; Lot prepared by the process of direct mingling, but indirectly. The method of preparing black flux is thus: intimately mix in a mortar two parts of cream of tartar with one part of nitre; project the mixture into a red-hot crucible; crucible, and allow the whole to cool. As the result of this treatment, you will eventually obtain an intimate mixture of Experiment (4.)--Mix a little charcoal powder with a little carbon (charcoal and carbonate of potash). Where does the more than its own weight of nitre :-place the mixture on a charcoal come from, you perhaps may inquire? Not from the slip of glass similar to the last, and apply heat as before. If nitre, certainly: this we all know. It must come, then, from sufficient nitre have been used, the charcoal will altogether the cream of tartar; but how or why, you are not in a position disappear, by the operation of causes, hereafter to be explained. to understand until we shall have, at a future period, detailed Now, it so happens that all animal and vegetable substances. the nature and the relations of carbon. One contradiction, contain a large quantity of charcoal carbon, although veiled. however, may probably occur to you; seeing that in a previous If, therefore, animal or vegetable bodies be mixed with nitre-operation we have availed ourselves of nitre for the purpose the mixture dried and ignited--that result which we have seen of burning away carbon. Why, you may ask, does it not do to occur with charcoal alone will occur with them-the carbon so in the present case? Simply because we have not used a 1 |