1834.] Silver on Muriatic acid gas, at high temperatures. 627 



The vapour of water being thus proved to be the principal agent, it 

 seemed evident that the muriatic acid gas must be decomposed at a 

 red heat by silver, although it is generally maintained that this metal 

 exercises no action whatever on the acid even at high temperatures. 



To ascertain this point, a slip of silver, rolled in a spiral, was placed 

 in a porcelain tube passing through the furnace. A current of mu- 

 riatic acid gas was admitted from one end, passing first through mu- 

 riate of lime to dry it ; at the other, a curved tube and chamber was 

 fixed, to collect the gas that might be disengaged. At first some hy- 

 drogen was given off*, but the disengagement soon ceased, and the 

 muriatic acid gas continued to pass without decomposition. On ex- 

 amination, the surface of the silver was found coated with a varnish of 

 chloride, which had prevented the further action of the acid gas. To 

 remedy this evil, the slip was enveloped in alumina : — the action now 

 went on better, though still slowly, and the chloride had penetrated 

 but a little way into the coat of alumina. In the next experiment, salt 

 was added to the clay, bringing it in fact to the composition of the 

 cementation mixture ; and now the decomposition proceeded with 

 rapidity, the salt favoring in a singular manner the spreading of the 

 chloride of silver through the porous substance of the alumina; pro- 

 bably owing to a combination between the two chlorides. 



I have dwelt at some length on the above series of experiments, 

 because they afford a beautiful application of scientific inquiry to a 

 rude and practical process which has been handed down and imitated 

 from generation to generation, without the least knowledge of the real 

 action of the materials upon one another ; and so apparently simple, that 

 chemists had hitherto neglected to examine it. Yet in this rude result 

 of the experience of ages, what a host of chemical operations are com- 

 bined, and how necessary is every step of the process : — the brick-dust 

 for instance answers a double object, first, to decompose the salt, and 

 thus cause the disengagement of the acid gas ; — and secondly, to 

 absorb with the aid of the excess of the salt, remaining undecomposed, 

 the chloride of silver as it is formed, and thus both to keep the 

 surface of the gold free for fresh action, and to prevent the loss of 

 the silver by evaporation, for the chloride is of itself very volatile. — 

 The porous nature of the pounded brick allows the passage and access 

 of the vapours, and thus gives it a preference over unburnt clay for the 

 object in view. Again the wood fuel, or in this country, the cow-dung 

 cake, giving abundance of aqueous fumes, is indispensable to the pro- 

 cess, while the small intensity of its combustion affords a regulated 

 heat so as not to endanger the melting of the gold, and its open 

 texture promotes the circulation of the moisture, through the pile 

 3 l 2 



