Nature of certain Solutions. 93 



Glucinum sulphate. — Same treatment as the preceding and 

 similar reaction. 



Ferric sulphate. — Same reaction. 



In all these cases the reaction was extremely well marked. 

 It indicates that these sulphates do not exist as such in 

 solution, but that a portion of their acid is set free. 



3. Alums. 



The alums, as might be expected, show reactions similar to 

 those of the sesquisulphates, with one notable exception. 



Potash alumina alum is always dissociated in solution. A 

 specimen made by combining pure aluminium sulphate with 

 potassium sulphate was recrystallized ten times with thorough 

 washing of the crystals. The presence of free sulphuric acid 

 was as evident after the last recrystallization as before. 



Ammonia ferric alum is also dissociated by solution. 



Potash chrome alum. — This alum differs remarkably from 

 the others. After two or three recrystallizations, which of 

 course must be made at low temperatures, the crystals may 

 be dissolved in water without dissociation. The test liquid 

 indicates that there is no free sulphuric acid present. 



It appears therefore that this alum alone of its congeners 

 exists as an alum in solution. 



The alums consequently form a series with varying pro- 

 perties, according to the nature of the sesquisalt present. At 

 the head stands chrome alum, perfectly stable in solution ; 

 next alumina alum, dissociated in solution but re-forming 

 itself by crystallization with the utmost facility ; next ferric 

 alum, also dissociated and also reforming itself by crystalliza- 

 tion, but with less facility. And finally manganese alum, 

 whose violet octahedra are so completely dissociated by solu- 

 tion in pure water that they cannot be reproduced by 

 crystallization. 



4. Acid Sulphates. 



At one time it was considered to be an established fact that 

 the acid salts of the alkaline bases were completely decom= 

 posed by solution in water into neutral salt and free acid. In 

 a review of this subject Berthelot * quotes Andrews, and 

 Favre and Silbermann, as expressing this opinion positively 

 and without question. His own investigation led him to the 

 conclusion that the separation was never complete, that a 

 certain proportion of acid sulphate was always present, 

 diminishing in proportion to the amount of water present, 

 increasing in proportion to the amount of free acid. The 



* Mech. Chim. ii. p. 318. 



