The solution of copper sulphate has now to be dealt with. It is therefore transferred, to a lead - lined tank, heated by a fire beneath, where it is evaporated down, and from which it is transferred into cooling - vats to crystallize. The crude salt, is again dissolved and recrys - tallized to prepare it for sale. The rooms in which all these operations are conducted are usually lofty and capacious, and ventilated by louvres at the roof.

In the nitric acid method, the granulated alloy is introduced into a platinum vessel of cylindrical form, like a show tea - canister, and 18 to 24 in. high, with a wide, short - necked opening at the top, into which a stoneware pipe can be fitted to carry off the fumes generated in the process, and another opening fitted with a cover, by means of which the solution is at the end of the operation poured off. At some works, the platinum vessel is provided with a short spout, through which the solution is poured out, and the vessel itself is furnished with pivots at the sides, which work upon iron supports, so that it may be tilted up to empty the liquid matters.

The gold alloy having been introduced and the nitric acid added, heat is applied below, either by a coke fire or by the flame of gas. At works in Sheffield where the nitric acid parting is performed on a small scale, large glass flasks, heated each upon a separate sand - bath, are used instead of platinum vessels. The operation of solution of the foreign metals occupies about 6 hours, during which time acid fumes are being given off. Part of the nitric acid is decomposed to oxidize the metals, with evolution of nitrous fumes, part unites with the oxides to form nitrates, and part comes off with watery vapour as nitric acid. Acid fumes are given out during the whole process of solution; but the nitrous fumes are most abundant in the earlier part of the process, and nitric acid fumes at the latter part. Acid fumes also rise when the contents of the platinum or glass vessel are poured out. The solution of the nitrates of silver, etc, poured from the platinum vessel or glass flask, is submitted to the decomposing action of copper in appropriate wooden vessels, and the solution of copper nitrate produced is reduced by iron or is sold to chemical manufacturers who may require it; or the impure silver solution is evaporated down, and crystallized and recrystallized so as to obtain silver nitrate in a sufficiently pure state for sale.

It is quite practicable to obviate nuisance, whichever process of " parting " may be in use. As respects the sulphuric acid process, the more essential requisites for the attainment of this object are efficient means of drawing off the vapours from the pot and the tank d, and proper means of disposing of them when drawn off. The draught of the chimney of the works may be used as the agent for the aspiration, or a jet of steam may be thrown into some convenient part of the pipe which conducts the vapours away, or the chimney draught may be aided by the steam jet. But the adaptability of the steam jet will in part depend upon the method of condensing the vapour which is preferred at the individual establishment. At Rothschild's and at Raphael's refineries, the acid vapour is condensed by means of cold, the object being to recover the evolved sulphuric acid as little dilute as possible. The method adopted is virtually the same at these two establishments. The arrangements for the first part of the condensation at Raphael's works are represented in Fig. 131. The pipe d is bent downwards towards the floor of the works, and in this vertical part is jacketed, cold water flowing through the jacket i.

This pipe terminates below in a leaden pipe j, about 16 in. wide and 20 ft, long, which lies horizontally in a trough of cold water. From this pipe any uncondensed fume passes into a leaden chamber, 10 ft. long, 4 ft. wide, and 5 ft. high, having 8 curtains arranged within it, so that the fume must pass alternately up and down in the chamber while traversing it from one end to the other. It is now conducted by a long pipe arranged round the interior of the workroom to a series of 3 leaden scrubbers, each about 6 ft. high by 3 ft. wide, and filled with large coke, which scrubbers the fume enters alternately at the bottom and the top, and, from the last of which, uncondensed gases pass away to a chimney 145 ft. high. At Rothschild's works there are no fewer than 12 leaden chambers distributed about the works, and in the cellars, as room could be found for them, each 6 ft. square; the fume has to pass through the whole series, and finally through 3 coke scrubbers similar to those in use at Raphael's works, on its way to the chimney, which is about 200 ft. high.

The acid collected in the various parts of these condensing arrangements is strong enough to make it worth while to concentrate it for use again in the iron pot (or about 60° Beaume). The concentration is effected at both the works mentioned by means of a cast - iron still with a leaden or platinum lining, and it is with a view to economy of acid and the utilization of the acid saved that the method of condensation described has been adopted. The sulphurous acid gas which passes into the chimney is discharged at too great an elevation to be any nuisance. At Rothschild's works, the tank c is closely covered down, and a pipe conveys the acid fume drawn off from it first through a condensing pipe surrounded with cold water, and then into the leaden chambers; it is in fact dealt with first in the same way as the fumes from the iron pot. In another establishment, the acid fumes from the pot, drawn off by means of a steam jet, are driven into a lead - lined covered tank filled with cold water, which effectually arrests the sulphuric acid; the uncondensed gases pass off from the tank into the chimney of the works.

In another case, the acid fume, first passed through a leaden pipe immersed in a trough of cold running water, is conducted through a small leaden chamber or scrubber packed with coke kept constantly wet with water, the aspirating force being the chimney draught. At both these last - mentioned establishments a use is found for the dilute sulphuric acid thus obtained, and nuisance is quite avoided. Over - heating and too violent boiling in the iron pot may be avoided by duly careful working. The escape of acid during the removal of gold from the large to the small pot is avoidable by waiting until the large pot has sufficiently cooled down, and by taking care that the small pot is not much heated before the gold is transferred to it. Where the same pot is used for both boilings, the fire should be drawn 1/2 hour before the solution is ladled out and fresh sulphuric acid is added. At Rothschild's refinery, a provision against nuisance from the accidental evolution of acid fumes from the pot into the atmosphere of the workshop, is made by enclosing the part of the apparatus where the pot is situated within a leaden closet or hood k. It is glazed on one side for the admission of light, and is open on the side next the working platform.