CHEMICAL SCIENCE. 26 



iron (when ferro- cyanide of potassium has been ernplo} - ed) and chloride of 

 gold. The latter is readily decomposed with formation of metallic gold ; 

 the other, at least partially, with separation of peroxide of iron, in fine 

 crystalline scales. Undecomposed chloride of iron, as well as chloride of 

 potassium, may be extracted with water after complete decomposition, for 

 which a slight red heat is sufficient; the gold forms a coherent spongy 

 mass ; the iron fine light scales, which are readily separable by mechanical 

 means. If any gold remain in the form of dust with the peroxide of iron, 

 it may be dissolved out with nitromuriatic acid, (the calcined oxide of iron 

 long resisting the action of the acid,) and the gold thrown down by 

 protosulphate of iron. In most cases this mode of separation will be un- 

 necessary. The author has convinced himself, by the employment of meas- 

 ured volumes of the same solution of gold, evaporation, heating with mu- 

 riate of ammonia, and so forth, that even the quantity 'of gold in such 

 solutions may be determined with sufficient exactness. 



The same process may be adopted with plating fluids ; chloride of silver 

 is obtained, together with oxide of iron, from the ferro-cyanide of potassi- 

 um ; the chloride is readily dissolved by ammonia ; metallic silver, of 

 which, however, but little or none is formed, is extracted by nitric acid. It 

 is unnecessary to say that the residue is operated upon in the usual man- 

 ner to obtain the silver ; nevertheless, as the decomposition of the plating 

 fluids may be effected in the humid way by means of sulphuretted hydro- 

 gen, this process may not be so frequently adopted for silver. 



Lastly, it may be useful to inform those persons who occupy themselves 

 with electro-plastic processes, that the employment of chloride of ammo- 

 nium or a salt of ammonia in this manner furnishes a ready means of 

 testing the composition of such fluids as are used in the formation of a 

 galvanic coating. For solutions of copper, the author employs sulphate of 

 ammonia, because, when muriate of ammonia is employed, chloride of 

 copper is formed, which is partially volatilized with the undecomposed sal- 

 ammoniac, producing a loss of copper. 



A NEW SOLVENT FOR COLLODION. 



MM. E. Mathew Plessy and Iwan Sehlumberger have proposed wood 

 spirit, or methylic alcohol, as a substitute for ether for dissolving collodion. 

 For this purpose it has many advantages ; as it is not so volatile as ether, 

 a thicker and more uniform coat can be applied on glass for photographic 

 purposes. The solution of collodion thus prepared is capable of dissolving 

 a much larger quantity of iodide of potassium than an ethereal solution, 

 and will consequent! y yield a more sensitive coating. The only incon- 

 venience attending the use of wood spirit, and which it is important to 

 notice, is, that, during its slow evaporation from the surface of glass, &c., 

 a certain quantity of formic acid is produced. By adding a little alcohol 

 of sp. gr. 40 to the wood spirit, and gently warming the glass plate upon 

 which the coating is to be put, the formation of the acid may be obviated. 



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