parts of water. Promote the disengagement of fluosilicic gas, which commences even in the cold, by exposing the flask or retort to a moderate heat in the sand-bath. Towards the end of the process a pretty strong heat should be applied. Every gas bubble ascending through the mercury produces in the water a precipitate of hydrated silicic acid. The rationale of this process is that, of every three equivalents of fluoride of silicon (Si Fl), one equivalent decomposes with two equivalents of water into silicic acid (Si O.), which separates, and hydrofluoric acid, which combines with the two undecomposed equivalents of fluoride of silicon, forming hydrofluosilicic acid. 3 Si Fl2+ 2 HO = 2 (H Fl, Si Fl) + Si O. The precipitated hydrate of silicic acid renders the liquid gelatinous, and it is for this reason that the aperture of the exit tube must be placed under mercury, since it would speedily be choked if this precaution were neglected. The same end may be attained also by attaching a funnel to the exit tube, by means of vulcanized india-rubber, and letting the funnel alone dip into the water. It sometimes happens in the course, and especially towards the end of the operation, that the gas forms complete channels of silica in the gelatinous liquid, through which it gains the surface without undergoing decomposition, if the liquid is not occasionally stirred. When the evolution of gas has completely ceased, throw the gelatinous paste upon a linen cloth, squeeze the fluid through, and filter it afterwards. Keep the filtrate for use. Tests.-Hydrofluosilicic acid, mixed with two parts of water, must produce no precipitate in solutions of salts of strontia. Uses.-Bases decompose, with hydrofluosilicic acid, forming water and metallic silicofluorides. Many of these are insoluble, whilst others are soluble; the latter may therefore by means of this reagent be distinguished from the former. In the course of analysis, hydrofluosilicic acid is applied simply for the detection of baryta. c. SULPHUR ACIDS. 5. HYDROSULPHURIC ACID (Sulphuretted Hydrogen) (HS). Preparation.-Hydrosulphuric acid gas is evolved best from sulphide of iron, which is broken into small lumps, and then treated with dilute sulphuric acid. Fused sulphide of iron may be procured so cheaply in commerce that it is hardly worth while to take the trouble of preparing it expressly. It may, however, be made, if necessary, by heating iron turnings or nails, from 1 to 14 inch long, in a Hessian crucible to a .white heat, and then adding small lumps of roll-sulphur until the entire contents of the crucible are in fusion. As soon as this is the case, the fused mass is poured out on sand, or into an old Hessian crucible. By making a hole in the bottom of the crucible in which the mixture is fused, the sulphide of iror will, as fast as it forms, run through, and may be received. in a coal-shovel placed in the ash-pit. Or, introduce an intimate mixture of thirty parts of iron filings and twentyone parts of flowers of sulphur, gradually and in small Fig. 22. portions at a time, into a red-hot crucible, awaiting always the incandescence of the portion last introduced, before proceeding to the addition of a fresh one. When the whole mixture has been brought into the crucible, cover the latter closely, and expose it to a more intense heat, sufficient to fuse the sulphide of iron. The evolution of the gas is effected in the apparatus illustrated by Fig. 22. Pour water over the sulphide of iron in a, add concentrated sul phuric acid, and shake the mixture; the evolved gas is washed in c. When a sufficient quantity of gas is evolved, pour the fluid from off the still undecomposed sulphide of iron, wash the bottle repeatedly with water, fill it with that fluid, and keep it for the next operation. If you neglect this, the apparatus will speedily become incrusted with crystals of sulphate of protoxide of iron, which is apt to interfere injuriously with subsequent processes of evolution of gas. For larger laboratories, or for chemists, having to operate often and largely with hydrosulphuric acid, I can recommend the lead apparatus designed by myself, which I have now for several years employed with the most satisfactory results in my own laboratory* (see Figs. 23 and 24). abcd and e f g h (Fig. 24) are two cylindrical leaden vessels, soldered with pure lead. They are both of the same size (in my own apparatus 13 inches high, and 12 inches in diameter). is a false bottom of lead, perforated like a sieve, placed about 2 inches above the actual bottom of the vessel, and resting on leaden feet, which support it on the sides as well as also more particularly in The apparatus is made by Mr. Stumpf, of Wiesbaden, mechanist, and fully answers all reasonable demands, both as regards workmanship and price. Fig. 23. the middle. The numerous holes in the sieve-like bottom have a diameter of 0.06 of an inch; k shows the opening through which the sulphide of iron is introduced into the vessel. In my apparatus this aperture has a diameter of 23 inches, and is closed by putting a greased leather ring on its broad smooth rim, and pressing down upon this by means of three thumb-screws, the broad rim of the smooth turned cover. shows the opening through which the solution of sulphate of prot oxide of iron is drawn off; it will be seen by the drawing that the bottom of the vessel (g h) slants towards the part where this opening is placed. The aperture has a diameter of about 1 inches; it is closed by means of a smooth-turned broad and thick leaden cap, fitting on the smooth-turned broad rim, and pressed down upon it with a thumb-screw. The semi 2 elliptical clamp which carries the female screw is movable, and hinged to the sides of l in a manner to admit of its being put out of reach of the liquid on drawing off the latter. The construction of the filling tube, m, may be learned from the drawing, and equally so that of the tube d h, which is intended to convey the acid from the upper to the lower vessel, and vice versa. It will be seen from the drawing that this tube reaches down into the slanting and deepened part of the bottom gh, without, however actually touching the latter. The tube c e is closed at the top, and has, therefore, no communication with the upper vessel, being simply intended to let off the gas evolved in e f g h; to which end it is connected laterally by a branch tube, with the tube o; this latter tube is fitted with a stop-cock (n). The tube q is closed at both ends, and serves simply as an additional support for the upper vessel. The tubes may have an inner diameter of half an inch. The process of filling is conducted as follows: put 3.3 kilogrammes of fused sulphide of iron, broken in lumps, through the mouth k, upon the perforated bottom i; screw the covers properly down upon k and I, shut the cock n, and pour through the funnel of m first 7 litres of water, then 1 litre of concentrated sulphuric acid, and then again 7 litres of water. The air in a bed escapes in this operation through p, even when the latter tube is already connected with the flasks 1, 8, t. If the cock n is now opened, and one of the cocks u, the acid will flow through the tube dl into e f g h; and through o air will escape at first, followed by the hydrosulphuric acid evolved in ef gh. As is seen in the figure, the tube o rises only to a certain elevation, when it makes a bend, running on thence in a horizontal direction. As many cocks, u u, are added as is thought desirable; these cocks are common brass gas stop-cocks, close-fitting and well ground in. They are connected with a small washing bottle; a double bent tube conveys the gas from the latter, with the cooperation of a straight tube connected with it by means of vulcanized india-rubber, into the fluid which it is intended to operate on; this arrangement greatly facilitates the cleansing of the straight tube dipping into the fluid. Upon now opening one of the cocks, u, the cock n being of course also open, you will at once obtain a current of gas of any strength desired, which will keep on for days in a continuous and steady stream. If all the cocks u, are shut, the gas evolved in ef g h forces the acid back to the upper vessel through the tube hd, and the evolution ceases. The cessation of the evolution of gas is not instantaneous, how. ver, as the sulphide of iron in e f g h remains still moistened with *The quantities here given are calculated for an apparatus of the dimensions stated. acid; moreover, small particles of the sulphides will always crum ble off, and dropping through the sieve, come into contact with the rest of the acid covering the bottom, g h. Now the gas which still continues to be evolved in e f g h, being no longer able to escape through o, forces the fluid up h d, and passing through the acid in a bed, makes its way out through p. To save this gas and keep it from poisoning the air, the flasks r, s, t are connected with p. r contains cotton, and serves the purpose of a washing bottle ;*s and t contain solution of ammonia; but the two flasks together should contain no more than either of them can conveniently hold; since, as the pressure of the gas increases or relaxes, the fluid is forced from s to t, or back from t to s. It will be readily understood that sulphide of ammonium is formed in these flasks. The evolution of gas ceases completely when all the acid is consumed, but there remains still the one half of the sulphide of iron, as the quantity used is calculated for double the amount of acid. The solution of sulphate of pretoxide of iron is therefore drawn off, and 1 litre of acid and 14 of water again poured in as directed. This apparatus is now made also of much less dimensions to adapt it for smaller laboratories. The apparatus described by Pohl deserves mention as being simple of construction and convenient in use. It is represented in Fig. 25. The bottle A containing the dilute sulphuric acid may contain from 1 to 3 or more quarts. In the caoutchouc stopper B, a stout glass-rod G, of at least of an inch in diameter, passes with considerable friction. Below, it bears a perforated basket, K, of hard rubber. It is lined with coarse linen and filled with lumps of fused sulphide of iron. When the glass-rod is so far depressed that the sulphide of iron just reaches the acid, a slow evolution of gas takes place. Fig. 25. The flow of gas is increased by immersing the basket, or checked by raising it above the liquid. The wide tube R, which forms a part of the delivery tube, is filled loosely with cotton and perfectly serves the purpose of a wash-bottle.f [In the Sheffield Laboratory of Yale Col lege the supply of hydrosulphuric acid is obtained from two large self-regulating * A common washing bottle filled with water could not well be used, as the water would very speedily recede. It This form of Kemp's gas generator is still more easily constructed by adopting |