LESSONS IN CHEMISTRY.-No. XIII.

THE metal which I purpose making the subject of this day's lesson is tin; a very interesting, and at the same time a very useful metal. No student, however remote he may be from towns, will experience any difficulty in obtaining a specimen of tin for examination. He may employ, to this end, a little tin-foil, or one of the capsules wherewith bottles of spirit, pickles, &c. are now so frequently occluded. I need scarcely remark that the metallic sheet known as tinplate, and used by tinmen, will not serve our purpose. This material is not tin, but iron coated with tin; however, supposing neither tin-foil nor a tin capsule to be procurable, which is hardly likely, the student may scrape off the superficial tin coating from a piece of tinplate.

The physical aspect of tin is very characteristic, so that, supposing this metal to be presented to you in the metallic state, you could scarcely confound it with any other. In the first place, it is a white metal; not blue-white, like zinc, but having more the appearance of silver. With lead it could not be confounded, on account of the bright aspect which it always preserves, whereas lead becomes tarnished. Tin melts with extreme facility, much more readily than lead; if held in the flame of a candle, it does not burn, as zinc does; neither does it oxidize, as is the case with lead similarly treated. In short, I repeat, tin in a metallic state can scarcely be confounded with any other metal; but you are aware that metals seldom exist in nature in the pure metallic state, hence the only way of distinguishing them and separating them is by taking advantage of their chemical properties. Under the head of antimony I mentioned indirectly a leading characteristic of the chemical demeanour of tin. I mentioned that this metal, like antimony, is violently attacked by nitric acid (aquafortis), a white insoluble powder remaining.

Let us try the experiment. Having placed a little tin-tinfoil by preference-in a watch-glass, saucer, or something of that kind, pour upon it a little nitric acid. Chemical action of a violent kind immediately ensues. The orange-coloured gas previously observed is again evolved, and oxide of tin remains. This result proves that the metal operated upon is either antimony or tin (p. 156, col. ii), and characteristics by which the chemist readily determines as between these two metals will soon be made apparent.

It may here be remarked, that very strong nitric acid does not readily act upon tin; if therefore the result as described does not immediately ensue, add to the nitric acid a few drops of water; you will then succeed.

From a consideration of the properties of tin just mentioned, its conversion into peroxide of tin by the action of nitric acid, it should follow theoretically that the peculiarity might be taken advantage of in analysis. This is indeed the case; the separation of tin from all metals, save antimony, by converting it into this insoluble powder (peroxide of tin) is an operation of frequent occurrence in analysis.

We will now take cognisance of the peroxide of tin under another phase. We will begin by dissolving the tin in a suitable menstruum, and we will convert the tin, thus dissolved, into an insoluble form. By this time you are aware, I assume, that chemists usually begin their analytical operations by converting into a solution the compound under analysis. There are exceptions to this proceeding, but I give you the rule. If a piece of glass were given you for analysis, you would begin by dissolving it; if a piece of compound metal, you would again dissolve it; if a flint stone, you would still proceed according to the same rule, you would dissolve it. There is a solvent for everything, even the hardest, the most intractable bodies; and a knowledge of the proper solvent for any given substance constitutes one of the most important parts of a chemical education. I cannot refrain, whilst treating of solvents, to direct your attention to one of the problems of the alchemists. These enthusiasts laboured hard to discover one universal solvent; in other words, a fluid that should be capable of dissolving everything wherewith it might come into contact. If such a liquid as this should be hereafter discovered, it

They forgot, by the way, the important fact, that, supposing the iquid in question were generated, a vessel would be required to hold it.

would be abhorred by chemists, and avoided by them to the utmost of their power. The presence of such a liquid would destroy all our means of analysis. We now effect the separation of different bodies by taking advantage of their several powers of solubility and insolubility, as you have seen in many cases and will frequently see hereafter. If all the substances which have come under our notice had been equally soluble in either of the fluids employed, there would have been an end to our powers of analysis.

To resume the special consideration of tin-hydrochloric or muriatic acid (spirit of salt), termed by the French acide chlorhydrique, is a very good solvent for the metal; still better is a mixture of hydrochloric with nitric acid, sometimes called nitro-muriatic or nitro-hydrochloric acid, also known as aquaregia, on account of its property of dissolving gold. As regards our present purposes, however, the generally best solvent for tin is not the best for us, the hydrochloric acid alone unmixed with nitric is what we will employ.

There are certain reasons, I will not stop to explain them just now, which involve the necessity of our performing this solution in a vessel of such construction that the minimum of atmospheric air may come into contact with the materials. It follows, therefore, that we ought not to effect the process of solution in an open vessel. A flask, therefore, is the proper apparatus to be employed; and inasmuch as one product of the solution will be a gas, the nature of which I should like you to investigate, let us adapt a perforated cork and a bent glass tube to the solution flask, causing the delivery-end of the tube to terminate just under the mouth of a jar or bottle, resting, as formerly described, on the shelf of a pneumatic trough.

For the performance of this experiment, a Florence flask will answer perfectly well, and a spirit-lamp flame may be employed to aid the decomposition. Care also should be taken that more tin is placed in the flask than there is acid to dissolve; otherwise we shall not get exactly the kind of solution we require.

teral product, the nature of which I shall not stop to explain, As concerns the gas developed and collected, it is a colla fully anticipating that the student will accomplish this by his own unaided efforts. When the operation of solution has ceased, label the flask proto-chloride of tin, and set it aside. Some chemists term it the proto-muriate or proto-hydrochlorate of tin, by which name therefore the student will sometimes find it denominated in books. Whether it be a proto-chloride or a proto-muriate, depends on the solution of a problem, and involves a very curious theory, concerning which chemists have argued a great deal to very little purpose.

What! the student will perhaps exclaim, does the boasted accuracy of chemistry come to this? Can you not determine the constituents of the solution of tin in spirit of salt? Form no hasty conclusion of the sort; we can tell accurately enough what constituents are there, but we cannot tell how these constituents are united amongst each other. Take an illustrative case: a certain number of gentlemen and ladies go into a church arm-in-arm; arm-in-arm they come out of church; but it does not therefore follow as a consequence of the evidence before you, that they sat arm-in-arm whilst in church, or that each couple had a separate pew.

Thus is it with many disputed chemical combinations, we put certain bodies together and they are lost to our view. Afterwards we get them out again, but the manner in which they arranged themselves whilst there, is a mystery to us. The solution of common salt in water, affords a very prominent example of one of these disputed facts. Common salt, if dried and separated into its elements, yields chlorine and sodium; therefore it must be a chloride of sodium; it cannot be hydrochlorate of soda, inasmuch as hydrochloric acid contains hydrogen, and soda contains oxygen, in common salt both these elements are wanting. Dissolve this salt in water, and th mystery begins. It may dissolve as thus:

Hydrogen

Sodium

Water

Hydrochloric
acid

Soda

}

Hydrochlo-
rate of
Soda
<

POISON!

BICHLORIDE OF MERCURY,

OR

Hg. Cl

Antidote, white of egg.

The solution will frequently be required as a test, therefore do not throw it away. Should you by some mishap swallow this amount of bichloride, you would die after the lapse of about an hour. If some ignorant person should apply the stomach-pump, the time would be less. If, however, imme diately on discovering your mistake, you were to swallow the whites of five or six eggs, you would live out the full number of your days, none the worse for the dose. Probably you will consider this fact worth remembering. You may furthermore remember, as a collateral fact, that white of egg is also an antidote for verdigris and preparations of copper generally. That moreover it is a material perfectly harmless in all cases; consequently, even though the kind of poison should not be known, you may always give white of eggs.

Apparently we have not very far advanced with our consiOxygen. deration of the metal-tin. Two points, however, in connection with it we have well determined. It is converted into an Whenever you meet with an ambiguous case of this kind, insoluble white powder by the action of nitric acid, and it remember well the fact that the accuracy of chemistry is not is dissolved by the operation of hydrochloric acid, yielding as a impugned thereby, Do not waste your time in mere ingenious collateral result a gas, the name of which I have not mentioned, arguments pro and con. People who do this are not imbued but which I expect you to determine. The problem related to with the true philosophy of chemistry, which prompts to the one of those truths already mentioned in the course of these lessons, and which will enable you, if you have been attentive, establishing of large physical generalisations rather than a contemplation of these nicely balanced disputes. Some people to solve it. I shall conclude this lesson by informing you, that are such creatures of mere detail that they cannot take a com-chloride of gold is made by mixing together two parts of nitric prehensive view of any thing. Give them a poem to read, acid with one of hydrochloric (by measure), and adding to this their first impulse is to hunt after stray commas, or determine fluid as much leaf gold as it will dissolve. Label the solution disputes of precedence between colons and semicolons. Give them chemistry to study, they are delighted with no part of it so much as the endless discussion about the aqueous decomposition or non decomposition of haloid salts, for thus chlorides, iodides, bromides and fluorides are termed.

All salts are termed haloid that result from the action of an acid containing hydrogen on any body. Thus chloride of tin is a haloid salt, inasmuch as it results from the action of hydrochloric acid on tin: in like manner, common salt (chloride of sodium) is a haloid salt, seeing that it results from the action of hydrochloric acid on sodium, or what amounts to the same thing, on soda. The term haloid is derived from the combination of two Greek words, aλç, salt, and sudos, likeness or similarity.

Returning now to the consideration of our solution of protochloride or protomuriate of tin (which you please), let us test its properties. For the purpose of testing, the following reagents will be necessary

(1.) A solution of carbonate of soda (washing soda). (2.) Of potash (liquor potassæ).

(8. Of ammonia (hartshorn).

(4. Of carbonate of ammonia (smelling salts).

(5.) Of bichloride of mercury (corrosive sublimate).

(6.) Of chloride of gold.

(7.) Solution of hydrosulphuric acid in water. (8.) Hydrosulphate of ammonia.

(9.) Some calomel.

CHLORIDE OF GOLD

OR

Au. Cl.

and preserve it as a test. Touch your skin with a little of this solution and observe the colour of the stain-developed by to-morrow, remember this result is indicative of gold.

And now one final word relative to the stain of chemical symbols referred to in this lesson. Bichloride of mercury has been represented in the symbol Hg. Cl. Now Hg. is the contraction for hydrargyrus (Lat. for mercury), and Cl. for Chlorine, the figure, expresses the fact that one equivalent of mercury or (200 parts by weight) combined with two equivalents of chlorine, or 36 parts by weight, gives rise to one equivalent of the bichloride of mercury.

As concerns the chloride of gold, you will observe it is simply termed chloride, without any numeral affix, because our auriferous liquid is a mixture of two distinct chlorides of gold (protochloride and bichloride) in variable proportions. If the solution were carefully evaporated by means of a water or steam bath, the result would be a chloride made up of three equivalents, 108 parts by weight, or of chlorium combined with one equivalent, or 200 parts by weight, of gold. This compound is called in exact chemical language a terchloride, and thus represented in chemical symbols:

Au. Cl3

Au., I need scarcely mention is the contraction for the Latin word Aurum, gold.

Two of these solutions, of bichloride of mercury and chloride made (protochloride) with hydrosulphuric acid, or hydrosulof old, require each special comment.

The former may be made of almost the strength of ten grains to two wine-glasses full of distilled water. The bichloride should be broken into fragments, projected into a Florence flask and boiled with the water. When cold, pour the solution into a bottle (glass stoppered by preference) and label the bottle thus ;

And now for two final experiments: test the solution just phate of ammonia, and remark, the colour is black, Next boil the protochloride with nitric acid, and then test it. The colour will be a sort of yellow, because the act of boiling with nitric acid converts the protochloride into a perchloride. All the other tests mentioned in our list affect solutions of tin. Let the student observe their re-action, more especially the effect of mixing bichloride of mercury with protochloride of tin.

Capillary Phenomena.-In the contact of solids and liquids, a series of phenomena are produced, to which the name of capillary phenomena is given, because that they are particularly observed in tubes, whose diameter is so small that it is comparable to the thickness of a hair.

The effects of capillary attraction are very various; but in all cases, they are the result of the mutual attraction of the liquid particles to each other, and to that attraction which subsists between these particles and solid bodies. Take, for example, the following phenomena: when we immerse a solid rod in a liquid which will wet it, the liquid, in opposition to the laws of hydrostatics, rises around the solid rod, as in the case of glass and water; and its surface, instead of being horizontal, takes a concave form, as shown in fig. 47; but, if a solid rod be immersed in a liquid which will not wet it, as in the case of glass and mercury, the liquid, instead of rising, sinks round the solid rod, and its surface takes a convex form, as shown in fig. 48.

These phenomena become more evident, when, solid rod, we immerse in the liquid glass tubes of meter, as shown in the following figures A and B. Fig. A. Fig. B.

Laws of Ascent and Depression in Capillary Tubes.--M. GayLussac has proved by experiment that the ascent and depression of liquids in capillary tubes, are regulated according to instead of a the three following laws: 1st. There is an ascent when the small dia-liquid wets the tubes, and a depression when it does not wet According them: 2nd, this ascent and depression are in the inverse ratio of the diameters of the tubes, so long as they do not exceed the tenth part of an inch: 3rd, the ascent and depression vary with the nature of the liquid and with the temperature; but they are independent of the substance of the tubes and of the thickness of their sides, if the latter be previously wetted. These laws hold good in a vacuum as well as in air.

The method employed by M. Gay-Lussac in the discovery of these laws was the following: 1st, he measured the interior

of the metal being known, it was then easy to deduce, from its weight and the height of the column, the required diameter, as shown in a former lesson: 2nd, he then placed the liquid under consideration in a vessel A B C D, figure G, and vertically immersed in it, the capillary tubes which were successively submitted to experiment; close by each tube, he placed a rod EF, tapering to a point, which, by the motion of a screw, was made to reach the exact level of the liquid; then, by means of a cathetometer, he measured the vertical distance between the upper extremity of the column of liquid in the tube, and the lower extremity or point of the rod which came in contact with the liquid. The heights which different liquids reach are by no means the same, as may be seen in the following table; for, in a tube whose interior diameter was about one twenty-fifth part of an inch, the liquids rose to the different heights here mentioned, above the level of the liquid in the vessel :

other so as to form an angle, and be immersed in a liquid which wets them, so that their line of contact be placed vertically, the liquid will rise towards the vertex of the angle between the two plates, and its surface, from the highest to the lowest point, will assume the form of the curve called an equilateral hyperbola. The asymptotes of this curve which is double, being traced on each plate, are the vertical straight line in which the edges of the plates meet, which is common to both, and the horizontal straight lines which determine the level of the liquid in which they are immersed, as shown by the dotted lines in the following figure H.

Fig. H.

When the line of contact of the two plates is horizontal instead of vertical, as shown in their sections represented in figs. 52 and 53, and the plates are placed so as to form a very small angle, a drop of water put between them is hollowed at both its extremities into a concave meniscus, as in fig. 52, and

Fig. 52.

•10

•12

14

Laws of Ascent and Depression between two Plates Parallel or Inclined.-Phenomena analogous to those presented by capillary tubes, are produced between two bodies of any form immersed in a liquid, when they are sufficiently near to one another. For example, if we immerse in water two parallel plates of glass so near each other that the two curvatures formed at their contact with the liquid, are united, it is observed: 1st, that the water rises regularly between the two plates, in the inverse ratio of the interval which separates them; and, 2nd, that the height of ascent for a given interval, is the half of that which would have taken place in a tube whose diameter is equal to this interval. If parallel plates are immersed in mercury, depression takes place, but according to the same laws. Fig. 51.

C

B

Mercury.

is attracted towards the vertex of the angle of the two plates; but if the liquid does not wet the plates as is the case with mercury, the drop of the liquid is rounded at both its extremities, into a convex meniscus, as in fig. 53, and is repelled from the vertex of the angle. The directions of attraction and repulsion in these figures are indicated by the arrow heads.

The force of attraction of a liquid to the sides of a vessel lies between two extreme cases; it is equal to that of the liquid to itself, or it is zero; in the former case, the ascent of the liquid in tubes is the consequence; in the latter, depression is the result. Between these two extremes, there must be the case in which there is neither ascent nor depression; this occurs when the force of the attraction of the liqaid to the solid is exactly equal to half of the force of the attraction of the liquid to itself. Water brought in contact with well polished steel appears to realise this particular case; for the liquid seems, on the approach of the metal, to experience neither elevation nor depression.

As already observed, every column of liquid elevated by capillary action is terminated by a concave surface; and every two plates of glass, A B and A c, fig. 51, be inclined to each column depressed, by a convex surface. In cylindric tubes of