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skill and experience, and costly apparatus. Qualitative analysis will occupy as much time and space as we have to fill, and even that will be pursued only so far as the means which I have supposed to be at the disposal of the student will admit.

In order to perform such operations as the analysis of the contents of organs of people who have died from poison, the analysis of earths, guano, and such like, it will readily be believed that long practice and laborious study are indispensable, and I cannot hope to teach in a few pages—or in any number of pages, without practical demonstrations- how to do these things; but as Rome was not built in a day, so do not skilful analysts become skilful all at once; and it is to that preparatory course, through which they, as beginners, have to go, that I propose to introduce my readers, hoping that they will see so much of the beauty of the science, and be so far interested in those operations of it which will be shown to them, that they will go on further with the study, pursuing it far beyond the bounds of this treatise.

I have already stated that by their identical behaviour with certain special reagents, certain elements mark themselves out as distinct from all other elements which do not behave in a similar manner, these others again marking themselves out into distinctive groups by reason of their identical behaviour with certain other special reagents. There arc, according to the division of Dr. Fresenius (whose two fine works on Qualitative and Quantitative Analysis arc well known), six of these groups, viz.'

t. Potassa, soda, ammonia .... (PtCl, and T).

2. Baryta, strontia, lime, magnesia . . . (NH.,O,Coo, and So3).

3. Alumina, chromium (Ko, and NHS,CI).

4. Zinc, manganese, nickel, cobalt, iron . . (NH ,S).

5. Silver, mercury, lead, bismuth, copper, cadmium (HCI,HS).

6. Gold, platinum, antimony, tin, arsenic . (HS).

The special reagents attached to these groups arc marked against each of them, and it will be well for the student to try the behaviour of each clement in the six groups, not only with these distinguishing reagents, but with the following general reagents, applied separately and successively in the order in which they are placed. Sulphuretted hydrogen or distilled water saturated with the gas; sulphide of ammonium; ammonia; potassa or soda; carbonate of ammonia; sulphuric acid; hydrochloric acid.

He should ascertain for himself, by actual trial, how the different elements behave on being brought into contact with the above-mentioned reagents. He should ascertain in what hquids precipitates produced by any of them are soluble; and for this purpose he should first try cold water, then hot water boiled with the precipitate in a test-tube over the spirit-lamp. Should the precipitate not yield to this treatment, he should add a few drops of nitric acid, and boil again, continuing to add acid until he has found means of dissolving the solid. He will find all precipitates, with the exception of one or two rare ones, yield to this method; but he'must be careful not to put in too much acid — it should be added in a few drops at first, and then additions made drop by drop. Some of the precipitates are soluble in an excess of the reagent that threw them down. This is especially the case with the oxides of most of the metals when thrown down from their solutions by ammonia or potassa; it is also the case with some of the sulphides thrown down from solutions by sulphide of ammonium. Carbonate of lime (CaO,CO„) is soluble in an excess of carbonic acid, as will be seen if—in the experiment already described, where chalk is thrown down from a solution of lime in water, by blowing with the breath into it through a glass tube—the blowing be continued. In this case, when the lime-water is so saturated with the carbonic acid gas blown into it from the lungs as to throw down all the lime contained, in the shape of carbonate of lime, it yields to the pressure of a continued in-draught of the gas, and takes up again in acid solution the salt to which it gave birth.

The behaviour of each element in the six groups, being manifested by the tests, should be carefully noted in a memorandum book. There should be stated whether a precipitate follows upon the introduction of a reagent, and if so, the colour and general characteristies of it should be described; its solvent should also be ascertained and mentioned.

By careful comparison of the results thus obtained, distinctions may be noticed, which will serve as means for the separation of the bodies so distInguished. To take a very simple case, suppose a solution which is to be .ested contains salts of silver, lead, and protoxide salts of mercury. To this solution, diluted with water, add hydrochloric acid drop by drop, shaking the test-tube after each addition, so as to mix the reagent and the test solution thoroughly. When no more white precipitate is thrown down, add to the mixture a large quantity of water, which must be decanted as the precipitate settles. Repeat this washing several times, and after the last time throw the contents of the test-tube on to a filter, catching the filtrate (the fluid that runs through is so called) in a test-tube for further examination. By means of the washing-bottle, wash well the residue on the filter, so as to free it from lead, and then dissolve the soluble part of the precipitate still remaining with ammonia. Ammonia will dissolve all the silver contained, and will convert the subchloridc of mercury into the black suboxide of mercury, which will remain on the filter. The filtrate will contain atl the chloride of silver, which can be again thrown down from solution by the addition of nitric acid in such quantity as to neutralize the ammonia and overcome its power over the silver chloride. In this way, then, the three members of the fifth group, which are similarly acted upon by hydrochloric acid (this acid will not so act upon the salts of any other elements), are distinguishable from the members of all other groups and may be distinguished from one another.

After the student has thus observed in practice the behaviour of the elements with the various reagents I have mentioned, he may proceed to the analysis of any inorganic matter, no matter how many substances there may be in combination; that is to say, he may analyse the bases of the substances, the analysis of the different acids being a delicate and difficult operation, to be undertaken only after repeated trial has perfected the student in his m2thod of detecting the bases. It is not possible, either, to give directions for the detection of them within the limits of an elementary treatise.

The following instructions being strictly and perseveringly attended to, will be a sure guide to the detection of any of those elements enumerated in the lists of groups. The reagents should be applied in the order in which they are arranged, that order being founded upon their observed effect upon the bases they are designed to detect. Let us suppose a mixture to contain all the elements, in some shape or other, which are included in the six groups. The first thing to be done is to pass through the slightly acid liquid a stream of sulphuretted hydrogen gas, which will be done through the medium of the gasgenerator already described.

Sulphuretted Hydrogen throws down as sulphides precipitates of antimony (orange), arsenic, tin (yellow), gold, and platinum (black), which arc soluble in sulphideoj'ammonium,and again thrown downfrom that menstruum by the addition of hydrochloric acid. It also, at the same time, throws down as sulphides: mercury, silver, lead, bismuth, and copper, in black or brownish black precipitates, and cadmium as a yellow precipitate. These latter are all soluble in sulphide of ammonium, so that here are two divisions already. We will deal with each of them presently.

Sulphide Of Ammonium.—To the filtrate from the precipitates thrown down from the acid solution by sulphuretted hydrogen add sulphide of ammonium; there will be thrown down as sulphides: nickel and cobalt (black), manganese (flesh-coloured), iron (black), zinc (white); as oxides: alumina and sesquioxide of chromium; and, in combination with certain acids only, baryta, strontia, and lime; and, in combination with phosphoric acid, magnesia— but these last four as salts. These precipitates we will also deal with presently.

To the filtrate from the precipitates thus thrown down by sulphuretted hydrogen and sulphide of ammonium add an excess of chloride of ammonium, and then add carbonate of ammonia. Baryta, strontia, and lime will be precipitated, leaving magnesia, potassa, soda, and ammonia to be looked for in the residue in the filtrate.

Let us now take in order the different results we have obtained.

The solution made by sulphide of ammonium is to be treated with a slight excess of hydrochloric acid. If a white cloudiness appear, it is probably due to sulphur, freed by the acid; but if a yellow or orange precipitate is produced, it may contain antimony, arsenic, or tin; if darker still, it may contain gold and platinum; but as these latter bodies are not likely to be in the beginner's experimental stock, I will leave them out of the question for sake of the others. Should antimony, arsenic, and tin be suspected, the precipitate obtained as above should be dried, and then fused in a crucible with nitre and carbonate of potassa, by which means they are converted into the oxides of their respective metals.

The fused mass, on being treated with cold water, will yield ar.seniate of potassa, sulphate of potassa, and a small quantity of antimoniate of potassa, and stannatc of potassa; while insoluble antimoniate of potassa and stannate of potassa will remain behind. In order to precipitate the small quantity of these two latter salts dissolved, neutralize the solution by the addition of nitric acid, when they will fall down. The filtrate is to be tested for arsenic acid by the addition of nitrate of silver. The residue which remains after exhausting the fused mass by water contains binoxide of tin, or antimonic acid, or both. It should be mixed with cyanide of potassium, and reduced to a globule on charcoal before the flame of the blowpipe. If the globule is brittle, it is antimony; if ductile, it is tin.

But if the globule contain both tin and antimony, it should be reduced (by nitric acid) to the oxides of those metals. On boiling these oxides with tartaric acid, teroxide of antimony is dissolved, and may be detected by sulphuretted hydrogen. Binoxide of tin is not affected by tartaric acid.

These metals, in consequence of their closely similar behaviour under the influence of the same reagents, are very difficult to separate, excepting after much practice. It will be seen from the foregoing that the task is no easy one. The student is advised not to try the analysis of them till after he has mastered the cither branches; but he may note their behaviour separately with the various reagents.

Boil in nitric acid the precipitate thrown down by sulphuretted hydrogen, but insoluble in sulphide of ammonium. A black residue is sulphide of mercury. Should there be a whitey precipitate floating in the liquid, it is of lead. Dilute with water, and add hydrochloric acid. The precipitate in this case will consist of chlorides of silver and lead, and subchloride of mercury. They should be treated as already described.

The filtrate from this solution may contain oxide of lead, the oxide of bismuth, oxides of copper and cadmium; and to it is to be added ammonia in excess. The precipitate thus obtained may be oxide of lead and teroxide of bismuth. It should be re-dissolved, and tested for lead with dilute sulphuric acid, and for bismuth by evaporating the solution to a small size, and then adding water. The filtrate from the excessive addition of ammonia, if of a blue colour, will indicate the presence of copper, which may be thrown down by ferrocyanidc of potassium. On the ammoniacal solution being neutralized by hydrochloric acid, carbonate of ammonia will throw down the cadmium.

The precipitate produced by sulphide of ammonium is to be dissolved in a mixture of nitric and hydrochloric acids (aqua regia), after which potassa in the cold is to be added in excess, and the mixture well shaken. The filtrate in this case will contain oxides of zinc and alumina, and sesquioxide of chromium. Boil this solution continuously, and sesquioxide of chromium will be thrown down; filter, and to the filtrate add sulphuretted hydrogen, sulphide of zinc will be thrown down; and the filtrate from that may be tested for alumina by saturating it with hydrochloric acid, adding ammonia, and digesting with carbonate of ammonia.

The precipitate left on the filter after the addition of potassa in the cold, as above, should be well washed, and then dissolved in aqua regia. To the solution add chloride of ammonium and ammonia. The oxides of cobalt, nickel, and manganese will remain in solution, and sesquioxide of iron will be thrown down, together with baryta, strontia, and lime if in combination with oxalic, phosphoric, or boracic acids, and, if in combination with phosphoric acid, magnesia also.

Add a few drops of acetic acid to the filtrate, pass sulphuretted hydrogen through it, and heat gently. The sulphides of nickel and cobalt will be thrown down, and if to the filtrate from them an excess of ammonia be added, and then sulphide of ammonium, sulphide of manganese will be precipitated.

The sulphides of nickel and cobalt should be dissolved in aqua regia, and thrown down as cyanides by cyanide of potassium, which will re-dissolve both precipitates; but (except under exceptional circumstances) hydrochloric acid added to this solution will throw down the nickel but not the cobalt. Borax dipped into a solution of cobalt, and fused before the blowpipe flame, gives a beautiful blue glass. In the outer flame, borax similarly treated with nickel makes a red-coloured bead, in the inner flame a grey bead.

The precipitate remaining when the oxides of nickel, cobalt, and manganese are dissolved out is to be dissolved in hydrochloric acid, and tested for iron with ferrocyanide of potassium, with which it will form Prussian blue.

The detection and separation of the alkalies and the alkaline earths then remains to be accomplished, and this brings us to deal with the result described four paragraphs back, where, by the use of carbonate of ammonia in the presence of an excess of chloride of ammonium, baryta, strontia, and lime have been precipitated; while magnesia, potassa, and soda were to be looked for in the filtrate. A portion of this filtrate, tested with phosphate of soda, will, if magnesia be present, throw down insoluble phosphate of magnesia.

To detect potassa and soda when in conjunction with magnesia, all the magnesia must be got rid of by adding a solution of baryta, or sulphide of barium, till a precipitate is no longer formed; filter and get rid of any excess of baryta by addition of dilute sulphuric acid; filter again and test for the alkalies.

Antimoniate of potassa (KO,Sb05) produces a white precipitate of antimoniate of soda (NaO.SbOj); and chloride of platinum (PtCL) produces in salts of potassa, in the presence of HCl and alcohol, an orange crystalline precipitate. In the outer flame of the blowpipe potassa salts impart a violet tint when soda is not present; soda imparts a distinctly yellow-coloured flame, by-which it can always be recognized.

To separate baryta, strontia, and lime, precipitate with dilute sulphuric acid the two former, neutralize the filtrate with ammonia, and throw down the lime with oxalic acid.

When baryta is in company with strontia, the carbonates of them should be dissolved in HCl, the solution evaporated to dryness, and the residue digested with strong alcohol. Chloride of strontium is dissolved and may be detected by burning the alcohol, the flame of which will be of a beautiful red colour.

I have thus laid down rules for the guidance of the student founded in the main upon the rules of Dr. Fresenius and Dr. Will, of the Giessen Laboratory. That they arc good guides I can gratefully testify, and the student cannot do better than submit himself wholly to their direction. There is no doubt they are intricate, and must be followed with much patience and perseverance, the only forces which will overcome the difficulties of this most interesting branch of chemistry; but, intricate as they are, they are as simple as such rules can be. Let the student test the truth of them by applying them to his practice in his own laboratory; but I would advise him to begin with the separation, according to these rules, of two or three bodies, gradually increasing the number until even so many as are above prescribed for can be mixed and analysed.

OPTICS.

Opties is the science which treats of the laws of light. Light may be defined as the agent which, operating through the eye, produces the sense of sight. This is no longer believed to be due to the impact of luminous particles on the retina of the eye, but to a wave motion communicated by luminous bodies to an ethereal medium which pervades all space. Bodies through which light passes, as air, glass, &c, are named transparent j those through which light does not pass, as wood or iron, are said to be opaque. Light travels in a uniform medium in straight lines. When a sunbeam is admitted into a darkened room through a small opening, the rays may be traced across the room in straight lines by means of the floating particles of dust, which reflect a portion of the light. When light radiates from a centre, its intensity

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