8 JOURNAL OF THE 
experiments there was one-half of an equivalent of sul- 
phuric acid for every equivalent of chromium sulphate. 
It was Kruger’ who first attempted to show in 1844 
the presence of free sulphuric acid in the green solution. 
He thought this was proved by the acidity of the layer of 
alcohol poured over the green solution. 
Baubigny and Pechard’ have shown that the alum has 
always an acid reaction even aiter purification by means 
of alcohol. Further they regarded the following experi- 
ment as proving a partial dissociation of the salt. To 
twenty cc. of a saturated solution of the alum, three- 
tenths gram of ammonia gas was added, and, after shak- 
ing, the liquid was neutral to methyl orange; after some 
time the liquid which had become green on the addition of 
the ammonia, yielded violet crystals with strong acid- 
reaction and the mother-liquor had become acid to methyl 
orange. 
Whitney‘ has also attempted to prove the presence of 
acid by physical methods. Sodium hydroxide and barium 
hydroxide respectively were added to the green solution. 
The addition of a base must lower the conductivity of the 
solution as long as free sulphuric acid is being neutralized. 
He found the minimum when he hid added one-sixth of 
an equivalent of sodium hydroxide to the chromium sul- 
phate, or one-third of an equivalent of barium hydroxide. 
No explanation was given of the variation in the results. 
He also claimed to have proved the presence of free acid 
by the catalysis of methyl acetate. Lastly, the inverting 
action of green solutions of chromium chloride, acetate, 
nitrate, and sulphate was tried upon sugar solutions. He 
came to the conclusion that in the case of the chloride and 
nitrate, two-thirds of the acid was set free on boiling ; 
of the acetate more than two-thirds,and of the sulphate less 
than one third. In our own experiments as to the relative 
1 Compt. rend., 115, 604. 
2 Ztschr. phys. Chem., 1896, 20, 40. 
