ment, and hoping that much good may accrue from its complete or and thanks; occasional queries of the kind he mentions may be addressed partial adoption by your students, I am, &c.-W. B. Norwich, Feb. 3rd, 1854. [We have inserted this letter, because we think the plan a good one, being simple and free from objections which we have had to some other plans which have been proposed. Of course the author has given us privately his name and address, a measure quite indispensable to any proposal of the same kind which we may receive.] ANSWERS TO CORRESPONDENTS. T. C. H. L.: We must refer you to previous remarks with regard to the Italian Dictionary. The matter will be duly considered. With regard to the Key in each number to the English-Italian Exercises of the previous week, it would facilitate the progress of some earnest pupils without the aid of a master, but at the same time further the idleness of by far the greatest number. To satisfy their curiosity, they would compare the Key with the exercises of the previous week, and consider themselves exempted from the task of translating them. A living language is not learned, like a dead one, merely for the purpose of reading, but also for the purpose of speaking and writing; and with regard to this latter most important object, the self-activity of the pupils must be kept up by all means in the power of a teacher. However, a Key of all the previous English-Italian Exercises will be given after the termination of each of the principal divisions of the grammar, and then we think without the above-stated disadvantage. The H. PITT (Birmingham): See our answer to T. K., Clerkenwell. problem written out without reference to any diagram, is passably well. Write to the Editor of the P. B. E. as you do to us.-J. D. (Birmingham): When the chord of an arc, and the radius of the circle are given, the chord of half the arc may be found without Trigonometry, thus: From the square of the radius subtract the square of half the given chord, and the square root of the remainder will be the distance of the centre of the circle from the middle of the chord; subtract this distance from the radius, and the remainder will be the sagitta (or distance from the middle of the chord to the middle of the arc); add the square of this sagitta to the square of half the given chord, and the square root of the sum will be the required chord of half the arc. Otherwise: subtract the square of half the chord from the square of the radius, and extract the square root of the remainder. Subtract this root from the radius, and multiply the remainder by twice the radius; then extract the square root of the product, and it will be the required chord of half the arc. The versed line is the difference between the radius and the cosine. See No. 71, part 18.-J. F. TREGS (Glasgow): Isolated is pronounced ice-o-lat-ed. T. K. (Clerkenwell): We wish his answer to the Four Ball question had been right; we have had many solutions, but it is not yet answered.UN ETUDIANT (Darlington): Most likely a Greek Dictionary, &c., will be published.-T. F. (Holborn): The difficulty which he meets with in regard to exercises in parsing, or any other exercises, would be met by his joining If a mutual instruction society, a thing we have often recommended. any six young men, or old men, or men of mixed ages, agree to meet and assist each other, they can supply the place of a tutor among themselves: and if any serious difficulty occurs, we are willing to become umpire. It is quite possible to become a correct Latin scholar, or any other scholar, by doggedly studying the Lessons in the P. E.; but the mutual instruction system would greatly enlighten and shorten the process. It would even greatly add to the power of the system, as well as to its interest and its humanity, if the six mutual instructionists would take some ragged and destitute juvenile delinquent from the streets, and try to instruct him and make him a better member of society. By endeavouring to instruct such a one, they would learn much better themselves, and they would see the human mind expanding under culture, like a drooping plant transplanted into a new and better soil. Oh the pleasure of doing good! They would be amply rewarded. Their mercy would be twice blessed; it would bless both them that gave and them that received the precious boon. Remember" that the soul be without knowledge is not good." 4 ECOLIER (Wisbech): We shall take up the subject of Architecture again, when we can get an opening in the P. E.-J. CHILD (Tenby): Logarithms when we can get room in the P. E.-INQUIRER (Bideford): We don't think much of the books to which he refers; as to the Elementary Bodies in Chemistry, we hold in our hands a work of some value, which gives a list of 64 with their names, equivalents, &c. These, of course, will be given in order, in the Lessons in the P. E.-A. B. (Carnarvon): We do not know any College where students are admitted gratis; but what connexion this has with being a British Consul or Ambassador, we do not see!-ToM (Dublin): See our concluding Lesson on Music.-OPIFEX (Eaton Square) will find a description and drawing of the Marquois Scales, or parallel ruler, in the P. E., vol. I., p. 49, col. i, fig. 5, and line 13 from the bottom.-COQUUS (London): There is no such thing as perpetual motion inventible by man. M. GUTHRIE (Liverpool, Box W. 90) wishes an accession to a society of young men who are engaged in carrying on a manuscript magazine. As young men best understand one another, we hope this intimation will incite some unfledged geniuses to come forward and aid the good cause by their contributions. We hope, however, that these contributions will not be like those which the Professor, who used to meet his students in his own chambers finely carpeted, instead of the class-room, requested them to leave at the door.-T. GRIFFITHS (Aberystwith): See pp. 137 and 213, vol. II., P. E.; also pp. 119, 295, and 323, vol. III.; and pp. 207 et seq., Vol. IV. U. J. CHEMISTE (Burnley): Right.-HO HYPOMENON, D. JOSEPH (Sunderland) and WM. KEYS, (Hythe): Three or four numbers will complete "Cassell's French Dictionary'; and it is expected to be finished in five or six weeks.-C. L. D. (Wales) should study at any one of the affiliated colleges of the U. of London he pleases; also matriculate, and take degrees, but say nothing about his domestic concerns.-D. H. (Driffield): His important communication is under consideration.-GRATUS (Bodmin): Right, to us. SIMPLICITAS (Wemyss): His attempt at the solution of the Mathematical questions for matriculation in the U. of L., is very fair; but a little more perseverance would have been attended with greater success.-PHILOS (London): Any one who wishes us to recommend a tutor, must give his name and address, and free us of expense in the matter.-J. Wood (Manchester), and YOUNG MAN (Duntocher): We can't advise them.-D. R. B. (Dundee): Vol. II. of "Cassell's Classical Library" is finished.-SECULARIST (Bp. Wearmouth) proposes the following question, which is very old, it:" An eight-gallon cask of water is to be divided equally between two but ingenious. As it may be new to some of our young readers, we insert persons who have no measure of any kind, but a three-gallon cask and a five-gallon cask. How is it to be done? MNEMONICS (Upper Stamford St.): We have never seen or heard of any permanent good resulting from the application of Mnemonics to the acquisition of Languages or the Sciences. Ruddiman's old Latin example appears to us to be the best rule: Memoria excolendo augetur, that is, "The memory is improved by exercising it."-J. C. (Salisbury): Study the Mathematics.-W. MAUNDRELL (Seven-oaks): "Borrowed a sum s for n years at r per cent., to be paid off by the annual payment of a certain sum u. What ought that sum to be?" Here, the sum u is an annuity. Now Dr. Thomson, in his "Arithmetic," p. 275, shows, that in order to find the entire sum due for an annuity remaining unpaid for a given period, compound interest being allowed on all the money thus remaining unpaid, (α, -1) u we must use the formula s = n ; where s, n, and u, have the values given above, and a1 = In order to solve the preceding question, then, we have only to reverse the formula by a simple Algebraic (α, -1) 8 Example: If the sum of £7,905 16s. 4şd. process, thus: u were borrowed for 30 years, at 6 per cent., what annual sum would pay it off in that time? Ans., £100. For £7905.8183 is the sum borrowed; 100+6 7905.81838.06 =1.06; and a1 = ai - 5.743491; therefore u = 5.743491-1 a = 100 30 = 100: whence the answer is, £100 must be paid yearly. LITERARY NOTICES. GRAND DOUBLE NUMBER OF THE ILLUSTRATED FAMILY PAPER." "Cassell's Illustrated Family Paper," No, 13, bearing date March 25, will This be a Double Number, containing 16 Pages, price Twopence. Double Number will be magnificently Illustrated, and will be sure to command the highest approval of all Purchasers. THE WORKING MAN'S FRIEND AND FAMILY INSTRUCTOR. Re-issue of this celebrated and popular Work. This Work is offered, for a limited period, at a reduced price. The set comprises seven volumes, crown octavo. For the excellency and general interest of its contents it is not be surpassed, it forms a library in itself, including Tales and Narratives by William and Mary Howitt, Miss Meteyard (Silverpen), and other writers of eminence; leading articles on topics of deep interest to the working classes; the celebrated letters of Martha Makepeace on Domestic Economy; Papers and Extracts on Science and Art; Parson Frank's Short Homilies; Original Biographies; Poetry, &c., &c. Also, in two volumes, the same form and size, "THE LITERATURE OF WORKING MEN," consisting of Prize Articles, Tales, Essays, and Poetry, written exclusively by Working Men. In order to enable individuals to obtain this valuable Series without inconvenience, the volumes will be issued Monthly, Is. each, in cloth boards. The First Volume will be ready with the Magazines for April next. It contains 360 closely-printed pages, including in its contents "The Wood-nook Wells," a powerful Tale, by Mrs. Mary Howitt; "The Pye Street Boy," by Miss Meteyard (Silverpen); the History of John Weldon; Cromwell and his Times; Letters on Household Economy, by Martha Makepeace; Biographies of Eminent Statesmen and Authors; Original Essays, addressed to the Working Classes; and a large amount of entertainment and instruction in Prose and Verse. The volumes will only be issued on the above-named terms to persons subscribing to the whole set. CASSELL'S LATIN DICTIONARY, BY J. R. BEARD, LL.D.-'he public are respectfully informed that the publication of this Dictionary has commenced, and that it will be issued regularly every week until its completion, which will be in about Twenty-six Numbers, THREEPENCE each, or in Monthly Parts, ONE SHILLING each. Part the First is now ready; Part the Second will be ready with the Magazines for March. CASSELL'S FRENCH AND ENGLISH DICTIONARY.-The FRENCH, and ENGLISH portion of this important Dictionary is now completed, and may be had, price 4s., or strongly bound, 5s. The ENGLISH and FRENCH portion is in the course of publication, and will be completed in about Twelve Numbers, THREEPENCE each. The entire Dictionary, forming one handsome volume, will be ready with the Magazines for April, price 9s. 6d. GERMAN CASSELL'S GERMAN PRONOUNCING DICTIONARY.—The ENGLISH Portion of this Dictionary is now ready, price 5s. in stiff covers, or 5s. 6d. strong cloth,-The ENGLISH-GERMAN Portion will be completed as quickly as possible, in Numbers, THREEPENCE each; and the entire Volume, strongly bound, at 9s., will shortly be issued. ON PHYSICS OR NATURAL PHILOSOPHY. No. XXIII. PNEUMATIC AND HYDRAULIC MACHINES. (Continued from page 320.) Pumps.-Pumps are machines employed to raise water, by suction, by pressure, or by the united effect of both: hence, their division into three kinds, the suction or common lifting pump; the forcing pump; and the suction and forcing pump, or lift and force-pump. Before the time of Galileo, who died in 1642, the ascent of water in suction-pumps was attributed to Nature's abhorrence of a vacuum; but we now know that the cause is the pressure of the atmosphere. called the ascension-pipe. At a second stroke of the piston, the same series of phenomena are reproduced; and after a few strokes, the water, following the air, finds its way into the barrel. The effect of the succeeding strokes is now modified. During the descent of the piston, the valve is closed, the compressed water raises the valve o, rises above the piston, which then lifts it up and causes it to rise by degrees to the upper reservoir. There is, after this, no more air in the barrel, and the water, urged by the atmospheric pressure, rises with the piston, provided that the top of the barrel is no higher than 34 feet above the level of the water in the lower reservoir, which receives the lower end of the suction-pipe; for we have seen, in a former lesson, that a column of water of about 34 feet balances the pressure of the atmosphere. In order to ascertain the proper height which may be given The Suction-Pump.-The suction or common lifting pump to the suction-pipe a, it should be observed that in practice has been already so far explained in a former lesson; but in the piston is never brought down exactly to the bottom of the order to make the subject clearer to some readers, we shall barrel, and that there is always below it a small quantity of explain a model adapted for lecture-room demonstration, as air having the force of the atmospheric pressure. Suppose the shown in fig. 112. This model is composed, 1st, of a cylin-space which this quantity occupies to be of the barrel. Then the air in this space expands as the piston ascends, and when it is at the top of the barrel, the tension of the air in the barrel is of that of the atmospheric air, according to Mariotte's law. The air of the suction-pipe, therefore, cannot be rarefied beyond this limit; and consequently the water, in this case, can only be raised to the height of 33 of 34 feet, that is, to the height of about 33 feet. This height is still too great, because the water must be raised by a certain quantity above the valve s; so that, in general, we cannot give to the suctionpipe a greater height between the limits above mentioned, than that of about 26 or 27 feet. Thus we see that in the suction-pump the water is first raised in the suction-pipe by the force of atmospheric pressure, and that the height thus obtained does not in general exceed 27 feet; but when once the water has passed through the piston, it is the ascensional foree of the piston which raises it, and the height to which it may then ascend depends only on the force which moves the piston. The Forcing-Pump.-Having explained the nature of the common pump, erroneously called suction-pump-seeing that the suction of the water, or its ascent in following the piston, is due simply to the pressure of the atmosphere, and that it would fail in so doing as soon as the column of water exceeded that pressure, the limit being at the utmost within 34 feet-we proceed to describe the operation of the forcing-pump, represented in fig. 113: a is the suction-tube, having its lower end 1 drical barrel made of strong glass, at the bottom of which is a valve s opening upwards; 2nd, of a suction-pipe A, which is immersed in the reservoir containing the water to be lifted; 3rd, of a piston with piston-rod which rises and falls in the barrel, the piston being perforated in the centre, and having the orifice covered at top by a valve o opening upwards. The pump-handle P is employed to put the piston-rod and piston in motion. When the piston is raised from the bottom of the barrel, a partial vacuum is formed below it, and the valve o remains closed in consequence of the pressure of the atmosphere; while the air in the suction-pipe a rises by its elastic force, raises the valves, and partly forces its way into the barrel. The air below the valve s, being thus rarefied, the water rises in the suction-pipe until the pressure of the liquid column so raised, added to the tension of the rarefied air remaining in the pipe, balances the external atmospheric pressure on the water in the reservoir. When the piston is lowered, the valve s is closed by its own weight, and prevents the return of the air from the barrel into the suction-pipe. The air in the barrel, being now condensed by the pressure of the piston, opens the valve o, and escapes into the atmosphere by the pipe above the barrel, which is VOL. IV. immersed in the water as before. p is a solid piston without a valve, which moves in the body of the pump c, by means of a lever as in the common pump. The air is withdrawn from the suction-tube as before; but instead of escaping through a valve in the piston, as it cannot return through the valve r, it is forced by the descent of the piston p up through the valve i, into the ascension-tubes; the water then follows by the pressure of the atmosphere and the ascent of the piston, and is forced through the valve r; this valve is then closed by the descent of the piston, and the water in the body of the pump is forced through the valve l, and up the ascension-tube s om which it cannot return, as its weight shuts the valve 7. This process is cninued until the water in the ascension-tube 101 received, reacts on the water and raises it in the tube D, unti? the piston re-descends; and in this manner, the jet continues without intermittance. is raised to the required height, the force necessary to raise it by pressure on the piston continually increasing until it reaches that height, and is discharged from the ascension-tube. In another form of the forcing-pump, there is no use made of the pressure of the atmosphere. The suction-tube, of course, is dispensed with, and the pump-barrel is itself immersed in the reservoir of water to be raised. The continuity of the jet of water from the ascension-pipe is obtained by the action of a vessel or reservoir of air, which we shall explain in the follow-distance of the orifice of the jet from the level of the water in ing description of the pump called the lift and force pump, and sometimes the suction and forcing pump, which differs but little from that just described, except in the application of the airvessel to produce continuity in the flow. This object is also accomplished by the operation of two pumps acting alternately, as in the construction of the common fire-engine. The Lift and Force Pump.-This pump raises water both by suction and by pressure, that is, both by atmospheric pressure and by mechanical pressure. It has a solid piston, and at the bottom of the barrel on the top of the suction-pipe, a valve 8 opening upwards, fig. 114. Another valve o, also opening The Load of the Piston.-In the pump just described. after the water has filled the suction-pipe and the barrel up to the mouth of the jet, the force necessary to raise the piston is equal to the weight of a column of water having for its base "the horizontal section of the piston, and for its height the vertical the reservoir from which it is drawn. Thus let н be the pressure of the atmosphere, h the height of the water above the piston, and ' the height of the column of water which fills the suction-pipe and the lower part of the barrel. The pressure above the piston is evidently H+, and that below the piston H-h', since the weight of the column h' tends to balance the pressure of the atmosphere. Now, the pressure H-h' tends to raise the piston; the effective resistance, therefore, is the excess of H+h, above H-h'; that ́s, hh', which was to be proved. Practical Application of Pumps.-In practice, the following rules are observed in the construction of pumps. The velocity of the piston is calculated to vary from six to nine inches per second. The area of the aperture covered by the valves is about half that of the barrel of the pump. The diameter of the suction-pipe, and of the ascension or discharge-pipe, is about two-thirds of that of the barrel of the pump. The stroke of the piston in large pumps varies from three and a half to five feet. In good pumps the loss occasioned by the time required for shutting the valves reduces the effect to about four-fifths of that produced by the piston. The following figures present different models of pistons and valves employed in the construction of pumps. Fig. 115, a piston packed with leather; fig. 116, a piston packed with hemp; fig. 117, a piston furnished with a single clack-va ve; fig. 118, a piston with a double-clack or butterfly valve; fig. 119, interior of the barrel of a pump, in the bottom of which a single clack-valve works; fig. 120, separate view of a single clack-valve; fig. 121, a conical valve. M. upwards, closes the aperture of a bent pipe, which, passing explanation of the forcing-pump, we repeat it here under another form, its importance demanding additional elucidation. The hydrostatic press is a beautiful application of a principle previously known for nearly two centuries, and commonly called the hydrostatic paradox; viz., that any quantity of water, however small, may be made to balance any quantity however great. The practical effect of (this principle is, that when water enclosed in a vessel quite full of the liquid, is pressed by a piston at any aperture with a given force, this pressure is at once communicated to every part of the vessel of the same size as the aperture, with the same force. Mr. Bramah, by an ingenious application of the forcing-pump to an apparatus constructed on this principle, produced one of the most powerful and useful machines used in the present day. It is represented in fig. 122, where s is the piston which moves in the cylindrical | succession; and let us suppose that the lower extremity of the tube g is placed between the two ajutages b and c. 1st. If we open first the ajutage b, the water runs out, its level is lowered in the tube g, and as soon as this level reaches that of b, the run of water is stopped. This phenomenon is explained by the fact of the excess of inward pressure which took place first at b; an excess which disappears when the level of the water in the tube g is brought down to b. For, before the water issued from b, the pressure on all points of the horizontal stratum be was not the same. Ate, it was composed of the pressure of the atmosphere and the weight of the column of water ge, whilst at b, the pressure was that of the atmosphere only. But as soon as the level of the water is the same at e and at b, there is an equilibrium, because that in the bottle and in the tube the pressure is then the same on all points if Fig. 122. tube f, or small barrel of the pump; p is the piston which moves in the cylinder co', or large barrel of the pump; and at bu is the tube of communication between the two barrels of the pump. A lever of the second kind raises the piston s, and the water in the reservoir b' is drawn into the barrel of the pumpf. When the lever is pressed downward, a valve shuts, and prevents the water from returning into the reservoir b', and forces it along the tube thu, in order to act upon the lower extremity of the piston p, to which is attached the plate p'; ef is another plate, against which the objects to be compressed by the machine are pushed by the former. In consequence of the quaquaversal pressure of the water forced into the large barrel of the pump from the small one, the pressure of one pound on every square inch of the surface of the liquid in the latter will be communicated to every square inch of the surface of the liquid in the former. Hence, if the diameter of the piston s be one inch, and that of the piston b be ten inches, the pressure of one pound on the former will be 100 lbs. on the latter. A noble specimen of this press was exhibited in the Crystal Palace by the Bank Quay Foundry Company, Warrington; viz., that which was used for raising the Britannia Tubular Bridge. The greatest weight lifted by this press was 1,144 tons, and the quantity of water used for every lift of six feet, was 81 gallons. The internal diameter of the great cylinder was twenty-two inches, and that of the ram or piston twenty inches. all directions in the interior of the bottle, according to the Mariotte's Bottle. The bottle of Mariotte is an apparatus principle of Pascal formerly explained, and the upper side k which exhibits some remarkable examples of the pressure of resists an upward pressure equal to H-ko; for the weight of the atmosphere, and by means of which we may obtain a con- the column of water ko partially counteracts the pressure stant flow. The neck of this bottle is closed by a cork, through which is transmitted to k. Now, according to the mechanical which a glass tube passes, open at both ends; in the side of principle, that reaction is always equal and contrary to action, the the bottle there are three apertures furnished with ajutages, pressure H-ko is urged downward by the side & on the stratum a, b, c, at right angles to the side and closed by wooden pegs.be; so that the particle of water at o, supports in reality two When the bottle and the tube are quite full of water, let us pressures, the one equal to the weight of the column of water consider the effect of opening one of the ajutages, a, b, c, in | ko; the other, the pressure н-ko, resulting from the reaction of the side k. The real pressure, therefore, which the particle | remembered that inasmuch as our desire is not merely to at o supports is ko+H-ko, that is, н the pressure of the atmosphere; which it was proposed to demonstrate. 2nd. If we shut the ajutage 6, and open the ajutage a, there will be no run of water; on the contrary, the air will enter the bottle by the aperture a, and the water will rise in the tube gpersalts of mercury generally-a solution of iodide of potasto the level ad, when the equilibrium will be restored. For it is easy to perceive by reasoning, as in the preceding case, that the pressure is then the same at all points of the horizontal stratum ad. discover a certain metal, but a given compound of a certain metal, we shall require not merely a test for mercury, but for that which is combined with the mercury and holds it in solution. Let us begin with a very characteristic test for soluble sium. The effects of this test are most remarkable, as will be seen. Having poured a weak solution of bichloride of mercury into a test-tube or a conical glass, add to it, drop by drop, another solution of iodide of potassium, and remark the beautiful play of colours which result, also the disappearance of all colour, all precipitate, and the resumption of perfect transparency in certain states of dilution; that is to say, so soon as a certain amount of test liquor has been added. It is useless to expatiate on changes which can be seen very much better than they can be described, but I may remark that by reversing the order of experiment in testing, and adding the bichloadding the latter to the former, the chromatic effects will vary. Now the peculiar effects here detailed are characteristic of a persalt of mercury; and the only persalt of mercury at all likely to come under one's notice in a case of poisoning is the perchloride of mercury; however, any doubt can be at once cleared up by the addition of the test for chlorine. Now what is the test for chlorine? You remember, I presume, that it is under the head of "Silver," come before us. Add, then, a little nitrate of silver solution, and a white precipitate results. But there are thousands of white precipitates; how shall we know what this one is? Simply by adding hartshorn (liquor ammonice) to it, the white precipitate dissolves. It has been produced, therefore, by chlorine. We have already determined the presence of mercury, therefore our substance must be a chloride of mercury. But as there are two chlorides of mercury, which is this? It cannot be the proto-chloride of mercury because that substance is insoluble; it must, therefore, be the per- or bichloride of mercury. Thus, at length, we arrive at 3rd. Let the apertures a and b be closed, and the aperture e be open. In this case, there will be a run of water with a conStant velocity so long as the level of the water in the bottle does not fall below the level of the lower orifice of the tube: for then the air would enter this orifice and fill the upper part of the bottle, where it would take the place of the water run off. In order to demonstrate that the flow of water at c is constant, it is necessary to show that the pressure on the horizon-ride solution to the iodide of potassium solution, instead of tal stratum ch is always equal to the pressure of the atmophere increased by that of the column hl. Suppose, then, that in the bottle the level of the water is lowered to the stratum ad. The air which has entered the bottle supports a pressure equal to H-pn. In consequence of its elasticity, the air transmits this pressure to the stratum c h. Now, this stratum supports besides this the weight of the column of water pm. Therefore, the pressure transmitted to m is in reality, pmHnitrate of silver-the reaction of that substance having already, -рn, oг н+mn, that is, нthi. In the same manner, it would be demonstrated that this pressure is the same, when the level is lowered to eb; and so on, so long as the level is above the orifice 7; the pressure on the stratum c h is therefore constant, and so is the velocity of the flow. But as soon as the level falls below the point l, this pressure decreases, and consequently the velocity of the flow diminishes. Thus, we see that the bottle of Mariotte is a means of obtaining a constant flow; namely, by filling it with water and opening the ajutage or aperture placed below the orifice of the tube. The velocity is then proportional to the square root of the height 7h, as shown in a former lesson. LESSONS IN CHEMISTRY.-No. XXII. In the course of any chemical examination, and more especially of metals, we find certain compounds demanding our especial attention. Amongst all the combinations of mercury, perhaps the bichloride, or corrosive sublimate, is that which claims our prominent notice. It is a terrible poison: its discovery and identification, when mixed with animal fluids, involve many points of chemical interest, and the manipulative processes employed in its extraction are of great beauty and delicacy. I must premise by directing the student's attention to the fact of there being two chlorides of mercury-one the protochloride, ordinarily known as calomel; the other, bichloride, ordinarily known as corrosive sublimate: the respective compositions of which are as follow: the demonstration. The next test we shall employ is ether; a liquid which, by the way, is rather to be considered as a separator than a test in the ordinary acceptation of the term. Having poured a little bichloride solution into a narrow test-tube, add to it about an equal volume of rectified ether (when the term "ether," without prefix, is used, chemists always mean sulphuric ether), then closing the tube with the thumb, agitate the tube. Proceeding thus, the ether and the bichloride solution will become intimately mixed. This mixture being effected, cork the tube (to prevent evaporation of the ether) and allow it to stand for the space of a few minutes at rest. Presently two distinct fluid layers will be recognisable; so well marked, so thoroughly individualised, that one may be readily separated from the other. The best means of effecting the separation is, as represented below, fig. 8, by the use of a little glass instrument termed a pipette. Hence the ratio of chlorine in these two chlorides is as one to two. Calomel may be generated in various ways. The student has already generated it by the addition of common-salt solution to proto-nitrate of mercury, and he will not fail to see, by reference to the preceding tabular exposition, that if by any process we can succeed in taking from a given weight of sublimate half the chlorine it contains, the result will be calomel; and if we can succeed in removing the whole of its chlorine, the result will be metallic mercury. Chemical investigation of Bichloride of Mercury in simple and omplex fluids.-Taking a portion of the solution of bichloride f mercury already prepared, let us master the appearances it The student, however, who does not possess the instrument affords with tests in simple solution. And here it will be may accomplish his result perfectly well by means of a glass |