104 On the Cause of Chemical Proportions. [Feb, 



oxygen equal to that necessary to convert it into an acid, it ouglit tn 

 contain 2i>'7 per cent, of oxygen. The above analysis, l!\ough it 

 only gives an approximation, proves, ho\vever, that in chioniio 

 acid the metal is combined with twice as much oxygen as in the 

 green oxide. If we take the synthesis of chromate of lead as the 

 wos-t exact experiment of those which 1 have given, and if we caU 

 culate from it the composition of the oxide and acid of chromium, 

 >ve obtain the following results ; — 



Green Oxide of Chromium. 



Chromium 70-24.. .100-00 



Oxygen , 29-76 .... 42-37 



100-00 



Chromic Acid. 



Ciiromium .54-13 100-00 



Oxygen 45-87 .... 84-/4 



100-00 



Now what is the number of volumes of oxygen contained in 

 each of these oxides ? Tiie composition of the chromates does not 

 allow us to conjecture that chromic acid is eitlier Ch + 2 O or 

 Ch + 4 O ; and as it is not probable that tlie green oxi;1e is Ch + 

 \4r O, the acid cannot l)e Ch + 3 O ; therefore no other nnmber 

 remains but Ch + 6 O. 



To throw some further light on this subject, I endeavoured to 

 obtain oxides containing less oxygen than the green oxide ; but 

 could not succeed. I exposed muriate of chromium previously 

 dried in a red heat to a strong heat in a retort, with the view of 

 obtaining oxymuriatic acid and a muriate at a lower degree of 

 oxidation than the green oxide ; but the experiment did not suc- 

 ceed I olitained at first a little muriatic acid ; then a pale red 

 substance sublimed, in form of small brilliant scales. The 

 greatest part of the substance remained unsulilimed, and was 

 slowly soluble in water. The sublimate was insoluble, and appeared 

 to be a submuriate of green oxide. 



M Vauquelin discovered some time ago a new oxide of chro- 

 niium, which, according to him, is intermediate between the green 

 oxide and the acid. He obtained that oxide by heating the nitrate 

 of chromium. It is obvious that the existence of this oxide can 

 only be explained by supposing the acid to contain C volumes of 

 oxygen. 1 resolved, therefore, to verify it. I dissolved in nitric 

 acid a portion of hydrous green oxide, and evaporated the solution 

 to dryness. The dried nitrate being slightly heated, swelled, 

 assumed a brown colour, and allowed nitrous acid to escape. I 

 removed a small portion at this part of the process, and dissolved 

 it in water. The solution had a brownisli red colour, very different 

 from that of chroraic acid. Caustic ammonia precipitated fron;\ 



