characteristic odor, the gas must be again passed through it until saturation is certainly attained.

In order to obtain still further assurance of the saturation of the liquid, it is often well to take the first portions of the filtrate of the succeeding paragraph and add a small quantity of sulphuretted hydrogen water (App., § 15). If a sufficient amount of the gas had not been passed into the liquid, the addition of the sulphuretted hydrogen water would cause the appearance of a precipitate. In such a case the filtered portion must be returned to the beaker and the stream of gas again passed through the liquid.

Pour the contents of the beaker, well stirred up, upon a filter which is supported over a test-tube or second beaker. Rinse the first beaker once with a teaspoonful of water, and transfer this rinsing water to the filter, allowing the filtered liquid to mix with the original filtrate. Label* this filtrate "Filtrate from II and III " (classes), and preserve it for later study.

If any considerable quantity of precipitate has adhered to the sides of the original beaker, it may be detached and washed on to the filter by means of a sharp jet of water from the wash-bottle. The precipitate, as it lies upon the filter, must then be washed once or twice with water; the wash-water

The student should at once make it a rule to label every filtrate or precipitate which he has occasion to set aside, even for a few moments. A bit of paper large enough to carry a descriptive symbol or abbreviation should be attached to the vessel which contains the liquid or precipitate. Paper gummed on the back or the small labels which are sold already gummed are convenient for this use.

This habit, once acquired, will enable the student to carry on simultaneously without error or confusion, several operations. He may be throwing down one precipitate, washing another, filtering a third. and dissolving a fourth at the same time, and the four processes may belong to as many different stages of the analysis. There will be no danger of error if labels are faithfully used; and a great deal of time will be saved. The unaided memory is incapable of doing such work with that full certainty, admitting of no suspicion or after-qualms of doubt, which is alone satisfy. ing, or indeed admissible, in scientific research.

ture of sulphide of copper (Cus) and tersulphi (As,S,). The fact that these sulphides are preci the conditions of this experiment proves that insoluble in weak acid liquors. They are also b in water. But an important difference between phides nevertheless exists, a difference which af worthy means of separating one from the other.

When the water has drained away from the pre the filter upon a plate of glass, and gently scrap tate off the paper with a spatula of wood or hor precipitate in a small porcelain dish (App., § 7 it enough of a solution of sulphydrate of sodium to somewhat more than cover it, and heat the tiously to boiling, stirring it all the time with The quantity of sulphydrate of sodium to be emp of course, with the bulk of the precipitate; in t or three teaspoonfuls will probably suffice. It is rable to use an unnecessarily large quantity of th reasons that will hereafter appear. A portion o precipitate remains undissolved; but a portion into solution. Filter the hot liquid again. The on the filter is sulphide of copper, which is insol in water and weak acids, but also in alkaline liqu filtrate, collected in a test-tube, add gradually acid, until the liquid has an acid reaction on (App., § 58). A yellow precipitate of sulphide o appear as soon as the alkaline solvent which k lution is destroyed. The sulphide of arsenic di sulphide of copper in that it is soluble in alkalin

In this series of experiments copper and arser as isolated elements, but as representatives of following common elements have sulphides which in water, weak acids and alkaline liquids: Le bismuth, cadmium and copper. These elemen Class II in our system of analysis. The follow

10

MERC RY AND LEAD.

988, 9 have sulphides which are insoluble in water and weak acids, but soluble in alkaline liquids :- Arsenic, antimony, tin (and the precious metals gold and platinum). These elements constitute Class III. If all the elements of both groups had been present in the original solution, one class might have been separated from the other by the same process employed in the case of the representative elements, arsenic and copper. The question may naturally suggest itself, how it happens that lead and mercury are included in Class II, when they were both precipitated in Class I. The chloride of lead, which is thrown down by chlorhydric acid, is not wholly insoluble in water; hence it happens that the lead is not completely precipitated in Class I. That portion of the lead which has escaped precipitation as chloride in Class I, will be thrown down as sulphide in Class II, for the sulphide of lead is insoluble in water, weak acids and alkalies. In regard to mercury, it will be remembered that there are two sorts of mercury salts, mercurous salts and mercuric salts. The mercurous chloride, Hg,Cl2, (calomel) is insoluble in water; but the mercuric chloride, HgCl2, (corrosive sublimate) is soluble in water. If, therefore, mercury be present in the form of some mercurous salt, it will be separated as mercurous chloride in Class I. If, on the contrary, it be present in the form of some mercuric salt, it will be separated in Class II as mercuric sulphide (HgS), for this sulphide is insoluble in water, weak acids and alkaline liquids. If a mixture of mereurous and mercuric salts be contained in the original solution, mercury will appear both in Class I and in Class II.

The treatment of Class II is fully discussed in Chapter III. The separation of Class III and the means of separating the members of the class, each from the others, form the subject of Chapter IV.

9. Definition of Class IV.-We now return to the study of the filtrate from Classes II and III. Pour the liquid into a small evaporating dish, and boil it gently for five or six minutes to expel the sulphuretted hydrogen with which the

expelled, hold a bit of white paper moistened w of acetate of lead (App., § 46) over the boiling the paper remains white, all the sulphuretted gone. Next, add to the liquid in the dish drops of strong nitric acid (App., § 4), and aga the liquid for three or four minutes, in order th present may be converted into ferric salts. 1 liquid into a test-tube, add to it about one thi chloride of ammonium (App., § 19), and finally water (App., § 16), little by little, until the being well shaken, smells decidedly of ammonia red precipitate of hydrated sesquioxide of iro from the liquid. Pour the contents of the te filter, rinse the tube and the precipitate onc water, and preserve the whole filtrate for sub tions.

Two other metals, aluminum and chromiu itated, as iron has here been, by ammonia-wa same conditions and in the same form, viz., These three elements, therefore, constitute the whose treatment forms the subject of Chapter drates of these elements are insoluble in wate presence of salts of ammonium, such as the ch monium which has been expressly added, and ammonium which has been formed during the of the acid liquid. The student may be cur why the presence of ammonium salts is insisted the elements of this class are thrown down by a The ammonium salts have nothing to do with tion of iron, aluminum and chromium; but by they prevent the precipitation, as will be hereaf of certain other elements whose hydrates, thoug soluble in water, are dissolved by solutions salts. The salts of ammonium are therefore a in solution certain other elements which oth encumber Class IV.

examination of the filtrate from the precipitate of Class Bring this liquid to boiling in a test-tube, and add sulp drate of ammonium (App., § 17), little by little, to the boil liquid as long as a precipitate continues to be formed. make sure that the precipitation is complete, shake the contents of the test-tube strongly, and then allow the mixt to settle until the upper portion of the liquid becomes cle Into this clear portion let fall a drop of sulphydrate of a monium; when this drop produces no additional precipita the precipitation is complete. Filter off the whitish prec itate of sulphide of zinc, and preserve the filtrate for furth treatment. It sometimes happens that this precipitate refus to settle and leave the upper portion of the liquid sufficient clear to test in the manner described above; in this case small portion of the mixture may be filtered and a drop sulphydrate of ammonium added to the clear filtrate in ord to determine whether the precipitation is complete. If no the filtered liquid must be returned to the flask and more the reagent added.

The element zinc, representing a new class of elements, precipitated under the conditions of the above experimen because its sulphide, though soluble in dilute acids, is insolu ble in alkaline liquids. The metals manganese, nickel an cobalt resemble zinc in this respect, and these four element therefore form a new class, Class V, in this analytical method The representative sulphide of this class was not precipitated by the sulphuretted hydrogen when that reagent was em ployed to throw down the members of the Classes II and III because the solution was at that time acid. Again it was not precipitated with Class IV by the ammonia-water, because the sulphuretted hydrogen with which the solution had previously been charged, was expelled by boiling before the ammonia-water was added. The complete treatment of Class V forms the subject of Chapter VI.