348 PREPARATION OF SULPHURETTED HYDROGEN. hydrosulphuric acid, which is sometimes called white, but which is in reality flesh or cream-coloured. Arsenic, cadmium, and tin (the latter in certain states of combination) are the only metals which yield a distinct yellow precipitate with it; the precipitate afforded by antimony is sometimes described as yellow, but it is in reality orange-coloured. Thus, by the evidence of hydrosulphuric acid the analytical chemist gleans a vast amount of information. Let us suppose the existence of an unknown solution. It has to be analyzed. It yields a precipitate with hydrosulphuric acid, and, therefore, contains a metal; moreover, it contains a calcigenous metal, and the metal is neither iron, cobalt, nickel, manganese, nor uranium, for these only yield a precipitate when an alkali also is present. Again, the solution is black, therefore the metal can neither be arsenic nor cadmium in any state, nor tin, in at least one state (i. e. as a persalt), nor antimony. What a vast amount of information then is conveyed by this one test! Preparation.-Direct union of sulphur with hydrogen has never yet been accomplished; but when sulphur is brought into contact with hydrogen in a nascent condition, combination between the two is readily effected. The preparation of hydrosulphuric acid is usually accomplished by acting upon sulphurets (sulphides) with sulphuric or hydrochloric acids. If hydrosulphuric acid be required absolutely pure, the most convenient sulphuret to employ is the sulphuret of antimony, and the best acid the hydrochloric (spirit of salt, or muriatic acid). The decomposition should be effected in a flask, heated by means of a spirit-lamp or other small flame; and the gas may be collected either over mercury or water-for although water dissolves from two and a-half to three volumes of the gas, the absorption soon reaches its maximum, and does not materially interfere with the pneumatic operation. It is but seldom, however, that the chemist requires to collect hydrosulphuric acid gas. In this and most other examples of the generation of a gas by the operation of heat on materials contained in a flask, it is proper to wash the gas in a little water: the means of accomplishing this will be soon described. Theory of the Decomposition.-When sulphuret of antimony is acted upon by hydrochloric acid the decomposition is as follows:-The chlorine of hydrochloric acid, by uniting with antimony of the sulphuret of antimony, generates chloride of that metal, whilst the hydrogen of hydrochloric acid combining with sulphur of the original sulphuret of antimony forms hydrosulphuric acid. These several grades of decomposition are more readily explained by means of the following diagram : 3 Hydrosulphuric acid 1 Chloride (sesquichloride) of antimony Sb S+3HC1 Sb,Cl3 + 3HS. In general laboratory operations sulphuret of iron is more frequently used than sulphuret of antimony for developing hydrosulphuric acid, and oil of vitriol diluted with about three or four times its volume of water is substituted for hydrochloric acid, If the sulphuret of iron be completely free from particles of metallic iron, the resulting hydrosulphuric acid is equally pure with that resulting from sulphuret of antimony *Stibium, the Latin word for antimony. Vide page 278. PREPARATION OF SULPHURETTED HYDROGEN. 349 and hydrochloric acid; but this seldom obtains; hence the gas is usually contaminated with a little hydrogen. The latter, however, is far from being prejudicial to the employment of hydrosulphuric acid-for most of its common applications is rather advantageous, by expanding the volume of the hydrosulphuric acid gas, and bringing it more fully into contact with the metal to be precipitated than otherwise would be the case. As the operation of preparing sulphuretted hydrogen is of frequent occurrence in the laboratory, I shall proceed to indicate more fully than I have hitherto done the proper apparatus to be employed. In the process already detailed, involving the use of sulphuret of antimony and hydrochloric acid, we found it necessary to use a flask and to apply heat; but when sulphuret of iron is the material acted upon heat is no longer necessary, and, consequently, a wide-mouthed bottle may be substituted with advantage for the flask. Thus modified, the apparatus will be as represented in the annexed cut. It consists of two wide-mouthed bottles; the greater of which may hold a pint, and the other need not be quite so another leading from the second bottle to the length of glass tube, which begins at the point marked, and extends down into a beaker glass, which is supposed to contain a metallic solution to be precipitated by the hydrosulphuric acid gas. If we desire to set this apparatus in operation, nothing more is required than to pour oil of vitriol properly diluted down the funnel F, when, on coming into contact with the sulphuret of iron, hydrosulphuric acid will be liberated, and the latter bubbling through the water in the small bottle will be washed free from any sulphuric acid which may by chance have come over with it; and hydrosulphuric acid will be finally driven through the solution contained in the beaker glass. The apparatus here described differs in some respects from that commonly provided for the purpose, and unquestionably with advantage. Firstly, it is usual to substitute for the straight pieces of tubing 1 and 2 pieces bent once at a right angle, effecting the junction between them by a very short connector of india-rubber. It is usual, also, to substitute for the compound tube of glass and india-rubber terminating in the beaker glass one piece of bent glass tubing. Now the advantages of the apparatus above represented are as follow:-Complete absence of rigidity is secured between the two bottles-a somewhat important matter, as the operator will now discover, if he work much in the laboratory; complete freedom of motion in the delivery tube, and, what is still of more consequence, the ability to join at the point a clean piece of glass tubing for every new operation. Let us suppose that the chemist has been throwing down arsenic by means of the rigid bent delivery tube usually employed for this purpose. Of course the tube becomes soiled with the solution internally, and until perfectly cleaned ought not to be used for the precipitation of any other metal. But to clean a glass tube bent into the form of a V is no easy matter, and in trying to do so either time is consumed (without avail sometimes), or the operator is tempted to prejudice his result by using a dirty tube. Our form of apparatus removes all these difficulties. One word now concerning the sort of india-rubber tubing to be employed. It must be vulcanized, as I have already stated, and vulcanized india-rubber tubing can be obtained of almost any dealer in chemical apparatus; but the exact kind of vulcanized tubing which best suits our present purpose must be sought in a quarter not very congenial, for the most part, to philosophic operations. It must be sought at the tobacconist's under the name of Hookah tubes. It is not only a cheaper material than that purchased at chemical warehouses, or even of india-rubber manufacturers, but it is better in several respects, smaller, more delicate, and more easy of adaptation. Sulphuretted hydrogen is either used as a mere qualitative test, to indicate the existence of certain bodies by the evidence of change of colour, or it is employed as a quantitative precipitant. In the former case it is generally preferable to employ a solution of the gas in water; in the latter the gas itself should generally be employed to avoid unnecessary dilution of the liquid acted upon. Water, to be impregnated with hydrosulphuric acid gas, should be pure and cold; it should, moreover, have been recently boiled to effect the expulsion of all traces of atmospheric air and other gases. Under these conditions water soon takes up its maximum of hydrosulphuric acid, the more speedily if it be agitated during the act of gaseous transmission. The solution thus obtained should be kept well corked and in a cool and dark place. Even with all this care it is decomposed in time, sulphur deposited, and the nauseous odour of rotten eggs, so characteristic of sulphuretted hydrogen, lost. Whenever these changes ensue the solution is worthless, and should be thrown away. Though the usual condition of hydrosulphuric acid be gaseous, it may be also obtained in the liquid and solid state. It is liquified at ordinary temperatures by a pressure of about sixteen atmospheres, being condensed into a transparent liquid having the specific gravity of 0.9. Its reduction to the solid state requires the simultaneous application of extreme cold and pressure. Sulphurous acid gas is not a supporter of combustion for ordinary combustibles, but it readily burns if a light be applied, the results of combustion being sulphurous acid gas and water. It is a remarkably poisonous gas, a small percentage of it in the atmosphere being fatal to animal life if breathed many times in succession. Still more extraordinary is the fact that an atmosphere contaminated with this gas can prove fatal to animal life by mere absorption through the skin. A French veterinary surgeon demonstrated this by enveloping a horse in a large air-tight bag containing this gas mixed with air. The horse died from the effects of cutaneous absorption. Nevertheless, the gas is exhaled from the hair of animals, and this fact is taken advantage of in the employment of a hair dye. Formerly nitrate of silver was employed as the universal hair dye, but it is attended with the great disadvantage of discolouring skin as well as hair. A mixture of oxide of lead and lime is now employed, and it has the advantage of dyeing the hair without discolouring the skin. The theory of the application is simply this :-Lead being one of the metals with which hydrosulphuric acid yields a black precipitate, and the hair being well penetrated with the metallic oxide, blackness results from the decomposition of that oxide by the hydrosul PRODUCTION OF SULPHURETS. 351 phuric acid which it meets with in the capillary tubes. The function of the lime is not only to effect removal of oil, but to render the oxide of lead somewhat soluble. It remains now for me to explain the nature of the precipitates thrown down by hydrosulphuric acid from certain metallic solutions. These compounds are sulphurets, otherwise called sulphides; the results of direct union, therefore, between sulphur and the metal, or other second body, as I have already explained, page 335. Consequently, when hydrosulphuric acid is passed through a solution of a metallic oxide, and a metallic sulphuret of the metal results, water also must be formed. The following diagram, illustrating the formation of sulphuret of lead by means of hydrosulphuric acid transmitted through acetate of oxide of lead, will render this statement evident :Acetate of lead (i. e. acetate of oxide of lead) Acetic acid Oxide of lead { Oxygen This production of sulphurets, by the action of hydrosulphuric acid on metallic oxides, is a very interesting subject for contemplation. Hydrosulphuric acid is the first hydrogen acid, or hydr-acid, that has hitherto come under our notice in this volume, and a slight amount of attention will assure us that its relations to bases are very different from the generally accepted relations of oxygen acids to the same. I say generally accepted, because chemists are by no means agreed concerning the exact scheme of the combination of the oxygen acids; but we speak of them at least as combining with bases, the latter being for the most part metallic oxides. Now let us contemplate the difference, as to scheme of combination, between the action of sulphuric acid and hydrosulphuric acid on one oxide-of lead for instance. If sulphuric acid be added to oxide of lead, or a solution containing it, a white compound (sulphate of oxide of lead, or sulphate of lead as, for brevity, it is sometimes called) is formed. Thus we have, or at least we appear to have, a direct combination of sulphuric acid and oxide of lead, thus: Sulphuric acid Sulphate of lead But if for sulphuric acid we substitute hydrosulphuric acid, the former type of combination is wholly departed from. We do not obtain hydrosulphate of oxide of lead, but sulphuret of lead and water. There can be no question about the formation of sulphuret of lead, because it falls and we obtain it bodily; but had it so happened that the result of transmitting hydrosulphuric acid through oxide of lead might have been a substance soluble in water, our chief evidence as to the formation of a sulphuret would have been absent. It might have dissolved as a sulphuret without decomposition of water, or as a hydrosulphate with decomposition of that fluid, as the two following schemes will explain. These remarks apply to all the hydracids without exception; and if we accept the Lavoiserian doctrine of salts, as being the results of union between acids (compounds) and bases (also compounds); and if, moreover, we continue to regard oxyacids as the type of normal acidity, the hydracids will constitute an intractable exception to such generalizations. But modern theory rather tends to the point of making hydracids the type, and associating salts of oxyacids with sulphurets, and other cognate bodies. There is no proper field in a volume of this kind for a full exposition of the arguments in favour of this doctrine or against it. I must, therefore, content myself with proving to the student by one example, that, the attempt to assimilate oxyacid salts with the type of hydracid salts, is not so impracticable as one might imagine. Take, for instance, sulphuret of lead. Its elements are sulphur, oxygen, and lead combined, as chemists usually assume, in the grouping of three of oxygen plus one of sulphur (sulphuric acid) with one of oxygen plus one of lead (oxide of lead), or simply thus : But it is also competent for us to assume the cxistence of a body having the following arrangement of elements : SO, Pb In which case the group SO4 would correspond in function with sulphur in sulphuret of lead. It would lead us too far from the objects of this volume were we to expatiate further on the theory of combinations; but, before quitting the subject, the curious fact may be noticed that if the strict notion of an acid, as adopted by Lavoisier, be advocated—if chemists see good reason for limiting the definition, salt, to the binary combination of a compound with a compound (an acid with a base), then must we be driven to the necessity of denying the claim of common salt to be a salt, inasmuch as both chemical analysis and chemical synthesis prove it to be a combination of the two elements, chlorine and sodium. Had it chanced to hold the elements of water in its crystalline structure, then we might have adopted one of two theories as to its constitution; we might have regarded it as a chloride plus water of crystallization, or as a hydrochlorate without, as the two appended schemes will illustrate— |