4 PROCEEDINGS OF THE AMERICAN ACADEMY. 



an accidental coincidence caused by an equilibrium which happens to 

 yield an approximately commensurate mixture. 



The present investigation was begun with the hope of discovering more 

 about the nature of the complex substances produced, and thus explain- 

 ing the mechanism of the change. The mode of attack consisted, first, in 

 the more persistent and thorough application of the devices of others, 

 and secondly of the introduction of other methods not used before on this 

 problem. Among the former dialysis, migration experiments, electrical 

 conductivity, and the distribution between two liquids were applied ; 

 among the latter, solid solution, catalytic sugar inversion, and the depres- 

 sion of the freezing point. While not a final unravelling of the tangle, the 

 research has succeeded in narrowing the possibilities of interpretation. 



The Preparation of Chromic Sulphate. 



The first problem to be solved was the preparation of chromic sulphate 

 in a pure state on a scale large enough to supply plenty of material for 

 the research. The sulphate was chosen for the research because when 

 once converted into the green mixture, it regains its violet condition far 

 more slowly than almost any other salt. Thus more time is available in 

 which to study the green solution. This very fact, however, introduces 

 a serious difficulty into the original preparation of the violet material, 

 which is much hydrolyzed by a very moderate rise in temperature. 



After sundry preliminary experiments which need not be recounted, it 

 was found most convenient to prepare the violet salt according to the 

 method of Traube (1848) with the additional precaution of careful 

 cooling. When alcohol is run into a diluted mixture of sulphuric and 

 chromic acids much heat is evolved, and the violet salt is accordingly 

 hydrolyzed. By immersing the vessel containing the reacting solution 

 in a freezing mixture and by adding the alcohol very slowly with con- 

 tinued agitation, the temperature may be kept below 10°, and less green 

 chromic sulphate is formed. Upon adding subsequently an excess of 

 alcohol the violet salt is precipitated in beautiful pale violet pearly 

 flakes, which may be further purified by solution in water and reprecipi- 

 tation by alcohol. We used an initial solution of chromic and sulphuric 

 acids containing about 200 grams and 300 grams respectively of these 

 substances dissolved in 600 grams of water. The yield of crystallized 

 salt was about twenty per cent of the theoretical yield. 



The salt thus obtained was analyzed for chromium by ignition and 

 for sulphuric acid by precipitation, taking care to use conditions which 



