1814.] On the Cause of Chemical Proportions. 105 



tills solution bulky brown flocks. The oxide thus obtained dissolved 

 readilv in sulphuric acid, forming a deep brown solution, which, 

 after being some time exposed to the light, became green. The 

 otlier portion of the nitrate of chromium was heated on a sand-batb 

 till it gave out no more nitrous vapours. I then poured more 

 nitrous acid over the brown mass, and continued the evaporation. 

 When it ceased to give out nitrous vapours I allowed it to cool. It 

 was now deep brown, rather brilliant, and mostly insoluble in 

 water and alkalies. What the water dissolved was only a portion of 

 brown nitrate not decomposed. The l)rown oxide thus obtained 

 was insoluble in acids ; but muriatic acid decomposed it with dis- 

 engagement of oxymuriatic gas. This intermediate oxide, then, 

 exists, and shows us that the acid must contain either 4 or 6 

 volumes of oxygen ; but as we have seen from the composition of 

 the chroraates that the acid cannot contain 4 volumes, it must of 

 necessity contain G volumes. Hence the known oxidi s of chromium 

 are, 1. Green oxide (oxidum chromosura), Ch + 3 O. 2. Brown 

 oxide (oxidun) chromicum), Ch + 4 O. 3. Chromic acid, Ch + 

 6 O. The volume of chromium, tiien. must weigh 708-045. 



But before quitting my experiments on cliromium, 1 think I 

 ought to state the following, though it has nothing to do with the 

 principal object of this memoir. The green oxide of chromium 

 undergoes combustion at a certain temperature, just as is the case 

 with several metallic stibiates and stibiites. Wlien the hydrous 

 green oxide of chromium is heated to a cherry red, it loses its 

 \<ater, and becomes deep green, almost black. If we now weigh 

 it, and then expose it to a strong heat, it appears to take fire, and 

 to bui;n an instant or two with a great deal of intensity. If we 

 \velgh it after it has cooled, we find that its weight is not altered, 

 but its colour has become a very beautiful light green, and it is 

 perfectly insoluble in acids. If we make this experiment with a 

 hydrate containing a small portion of subsulpliate or subnitrate of 

 cluomium, the acid remains combined witii the oxide till the 

 moment of incandocence, when it is disengaged, and constitutes a 

 little smoke, and in this case the oxide loses a little of its weight ; 

 but even in experiments when this smoke was very perceptible the 

 oxide did not lose more than ^ or i of its weight by the ignition. I 

 wish to draw tiie attention of the reader to this circumstunce, that 

 when the plienomenon takes place in the stil)iates it consists in the 

 •action of compound bodies (the oxide of antimony and the base), 

 while in the present case it consists in the action of elementary 

 bodies (cliromium and oxygen). Sir Humphry Davy has observed 

 an analogous phenomenon in the hydrate of zirconia, an.l he 

 a-;cril)e6 It to the incrciised colicsion of the parts of the earth at the 

 instant that the water separates from them. According to Mr. 

 J'^dmund Davy, the precipitate obtained from the miniate of pla-' 

 tJinim by sulpliuretcd iiydrogen produces tlte same phenomenon, if, 

 tfter being dried in an atmosphere dtstitute of oxygen, it be heated 

 in a retort cxliaustcd of air. It emits a little sulphurous acid gas 



