ElylSHA MITCHBI^Iv SCIENTIFIC SOCIEiTY. 7 



Upon the second of the primary, that is, the cuprous 

 sulphide produced. If the experiment were carried 

 out so as to cause a rapid evolution of gas and the resi- 

 due not allowed to form a protective coating- over the 

 copper, as long- as an excess of the metal was present, 

 only the primary reactions occurred. This was accom- 

 plished at 160"-170° C. If the strips of copper were 

 touching- they almost always became bound together by 

 the anhydrous copper sulphate and a coating of the 

 black residue formed a protective covering- to the cop- 

 per. When such a state of affairs occurred, no sharply 

 defined line could be drawn to show, of these second- 

 ary reactions, when the first ends and the second 

 begins, because as soon as some cuprous sulphide is 

 changed to cupric sulphide, the latter is attacked by 

 the sulphuric acid, sulphur being one of the products 

 of the last reaction. Several experiments carried out 

 at 140''-150"C. when this occurred with an excess of 

 copper gave evidence of all the reactions, primary and 

 secondary. Sulphur was deposited on the sides of the 

 flask and the black residue contained 20.71 per cent, 

 sulphur, and the theoretical percentage for cuprous 

 sulphide is 20.138. This showed the presence of some 

 cupric sulphide in which the percentage of sulphur 

 is 33.592. 



Some freshly prepared cuprous sulphide was treated 

 with concentrated sulphuric acid. Sulphur was deter- 

 mined in the undissolved residue, the free sulphur being 

 first removed. 



Found. Calculated for CuS. 



Sulphur 32.36 33.59 



The formula. 



^ Watts (vol. II, p. 41, 1875, Ed.) notes this complete decomposition. 



