228 Transactions of the Society. 



copper sulphide the author devised another and more conclusive 

 application of micro-chemistry. This test is based on the insolu- 

 bility of cadmium sulphide in potassium cyanide and the solubility 

 of copper sulphide in the latter. A solution is prepared (equivalent 

 strength) of the double cyanide of cadmium and potash, to which 

 is aided a slight excess of potassium cyanide. On applying a drop 

 of this reagent to a dendritic growth containing sulphide of copper 

 the latter is dissolved, but each molecule of copper sulphide dis- 

 places from the solution an equivalent molecule of cadmium com- 

 bined with the sulphur derived from the solution of the sulphide, 

 with the result that in the case of some recent dendrites it is 

 possible to obtain, as a result of the reaction, a brilliant yellow 

 dendrite of cadmium sulphide which is an exact pseudomorph of 

 the copper sulphide. 



This principle of replacing one water-insoluble substance by 

 another with the formation of pseudomorphs differing in colour 

 appears to be capable of further applications and constitutes a new 

 mode of reaction in micro-chemical manipulation. 



