498 Mr. J. Denham Smith on the 



with excess of potash, washing the brownish-black precipitate 

 with hot water until free from alkali, and boiling this with 

 250 grs. of sulphate of copper, a light green-coloured powder 

 was produced, the boiling and digestion were continued for 

 forty-eight hours, and the liquor was then evaporated to dry- 

 ness. The mass was treated with water to dissolve out uncom- 

 bined sulphate, and washed until free from soluble matter; 

 this when dried gave a pale green powder weighing 172 grs. : 

 of this — 



36*32 grs. gave 23*8 grs. of sulphate barytes = 7*35 of sul- 

 phuric acid in 32*52 grs., which quantities afforded by igni- 

 tion 21*56 grs. of oxide of copper; this indicates the formula 

 S0 3 3CuO, 2HO, or 32*52 grs. consist of 



Theory. Experiment. 



Sulphuric acid ... 7*31 7*35 



Oxide of copper . . . 21-94 21*56 



Water 3*29 3*61 



32-52 32*52 



This salt is also obtained when less than an equivalent of 

 oxide of copper is boiled with an equivalent of sulphate of 

 copper. 



In Dr. Kane's tabular view of the sulphates of copper 

 (Transactions of the Royal Irish Academy, &c, vol. xix.), 

 this salt, the trisulphate of copper, is not noticed ; but Ber- 

 zelius has described one, assigning to it 3 equivalents of water. 

 If such a salt exists I have been unable to obtain it ; and 

 although I admit that such a composition is by no means im- 

 probable, seeing that another subsulphate exists in which the 

 number of equivalents of water and oxide of copper are equal, 

 I am inclined to consider that the third equivalent of water 

 in Berzelius's salt was hygrometric, as in no one instance, 

 although this salt was prepared at several distinct periods and 

 in the mode described by Berzelius, did I obtain results indi- 

 cating an approximation to the constitution S0 3 CuO, 3HO. 

 The composition I have assigned to this salt agreed with 

 that quoted by Dr. Thomson, as arrived at by Brunner from 

 the analysis of the subsulphate prepared by Berthollet's pro- 

 cess. 



Tetrasulphate of Copper. — This salt is obtainable in a great 

 variety of ways. It is precipitated when a cold solution of 

 sulphate of copper is mixed with an insufficient quantity of 

 carbonate or of caustic soda, or potash to completely decom- 

 pose it. It may be prepared by digesting together cold, — equi- 

 valents of sulphate of copper, and of well-washed precipitated 

 oxide of copper ; by adding a solution of potash to a warm 

 solution of sulphate of copper until a greenish-blue precipi- 



