and on a New Means of investigating them. 483 



2. Sesquisulphates. 



Chromic sulphate. — The violet salt was obtained free from 

 green salt by acting on the nitrate with sulphuric acid. It 

 was freed from excess of acid by repeated precipitation with 

 alcohol and thorough washing. It had a pale violet color. and 

 satiny luster. In solution it always gave when examined with 

 the test liquid, marked indications of the presence of free sul- 

 phuric acid. 



Aluminum sulphate. — This was purified from excess of 

 acid in the same manner as the preceding and gave a similar 

 reaction. 



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 aluminum 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 indi- 

 cates 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 proper- 

 ties, 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 reforming itself by 

 crystallization with the utmost facility; next ferric alum also 

 dissociated and also reforming itself by crystallization but with 

 less facility. And finally manganese alum whose violet octa- 

 hedra are so completely dissociated by solution in pure water 

 that they cannot be reproduced by crystallization. 



