500 Mr. J. Denham Smith on the 



the water, rapidly evolving heat, and regains its original pro- 

 portion, and also its proper colour." 



There exists another hydrate of the tetrasulphate, obtained 

 by precipitating a very dilute solution of sulphate of copper 

 by a solution of potash, also much diluted, adding the alkali 

 until the supernatant fluid restored reddened litmus to blue ; 

 this when dried was an extremely light powder of a very pale 

 blue colour, altogether differing in appearance to the other 

 tetrasulphates ; 18'63grs. of this salt gave 8*28 grs. of sul- 

 phate of barytes, and 29*38 grs. afforded 19*33 grs. of black 

 oxide of copper, indicating the formula S0 3 4CuO, 5HO, 

 or 



Sulphuric acid . . 

 Oxide of copper 



Water 



29*38 29*38 



Pentasulphate of Copper. — This salt is obtained when potash 

 is added to a solution of sulphate of copper, until the alkali 

 is slightly in excess and a light blue-coloured precipitate is 

 obtained ; this, unless rapidly washed, becomes gradually 

 dingy, and finally turns to a dark greenish-black colour, pro- 

 bably owing to the loss of combined water. This change often 

 takes place, entirely or partially, during the drying of the 

 precipitate, even when dried by exposure to air, without arti- 

 ficial heat; when obtained free from this blackening effect it 

 is a light powder of a blue colour, a more decided tint than 

 that of the tetrasulphate with five equivalents of water; upon 

 analysis, which was frequently repeated, the constitution of 

 this salt was found to be SO a 5CuO, 6 HO. The results of 

 two analyses of this salt, prepared at different times, are sub- 

 joined : 32*2 grs. of the salt gave 21*6 grs. of oxide of copper, 

 and 22-36 grs. gave 9*74 grs. of sulphate barytes, or 



Theory. Experiment. 



Sulphuric acid . . . 4*38 4*83 



Oxide of copper . . 21-90 21*60 



Water 5'92 5*77 



32*20 32-20 



24*3 grs. gave 16*47 oxide copper, and 17 grs. gave 6*52 

 grs. sulphate of barytes, equal to 



Theory. Experiment. 



Sulphuric acid ... 3*31 3*2 



Oxide of copper . . 16*53 16*47 



Water ...... 4*46 4*63 



24*30 24-30 



32*2 grs. heated on a sand-bath until it assumed an olive- 



