10 JOURNAL OF THE 
but basic (2Cr,O,.5SO,). These he refers to three condi- 
tions of chromic hydroxide: 'Cr,(OH),, precipitated by 
alkalies from violet chromic salts; Cr,O(OH),,, corres- 
ponding to the basic sulphate and non-isolable; Cr,O(OH),, 
precipitated by alkalies from green solutions. Solutions 
of this latter in acids, he says, are not precipitated by 
solutions of barium salts. This last statement is not 
strictly true. Such solutions are partially precipitated 
by barium salts just as all green solutions are. He as- 
signs to the green sulphate the formula Cr,O,.3SO,.11H,O, 
which he says has quite a different constitution from that 
’ of the violet sulphate. Proofs for this statement he does 
not give. Thisisomeric green sulphate, he says, possesses 
neither the characteristics of a sulphate nor of a chro- 
mium salt. One molecule can combine with one molecule 
of either sulphuric acid or a metallic sulphate. Thus we 
may have Cr,(SO,),.H,SO,, or Cr,(SQ,),.K,SO,. In these 
compounds all the sulphuric acid is in a non-precipitable 
form, he maintains. Hiscrucial experiment is as follows: 
‘*Mix a solution containing one molecule of Cr,(SQO,), 
with a solution containing one molecule of sulphuric acid 
or asulphate. Union is immediate, for barium chloride 
will now give no precipitate and the solution therefore 
holds no sulphate. The new radical is unstable, for pre- 
cipitation is immediate on boiling, or in concentrated solu- 
tions or in dilute selutions on standing one-half hour. /¢ 
7s necessary to work with very dilute solutions, otherwise 
the radical containing chromium %7s decomposed.” 
Of course such reasoning would give us these same 
strange isomeric metal-sulphuric acids in all sulphates, 
for if they are diluted enough it will take half an hour or 
more for the precipitate to form with barium chloride. 
On such slender basis Recoura builds up a series of 
salts of a hypothetical chromo-sulphuric acid (Cr,.4SQO,),. 
1 Compt., rend., 113, 1037 ; 114, 477 ; 116, 1367. 
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