198 PROCEEDINGS OF THE AMERICAN ACADEMY 



expected that a considerable quantity of these salts would adhere to the 

 precipitate ; but since all these salts are fixed at a low red heat, they 

 were not altered by the heat to which the precipitate was exposed. 

 The rest of the precipitate must have lost by the ignition all its water 

 and all the oxygen over and above that necessary to the constitution of 

 chromic oxide. By subtracting the weight of the water collected from 

 the total loss by ignition, the weight of the oxygen expelled is obtained. 

 The ignited residue was then washed out of the bulb of the reduction- 

 tube, digested in hot water, thrown upon a filter and washed with hot 

 water till only pure chromic oxide remained ; lastly, this oxide was 

 ignited and weighed. By subtracting the weight of the chromic oxide 

 from the weight of the whole ignited residue, the weight of the soluble 

 salts which adhered mechanically to the precipitate was obtained. The 

 following are the figures of an analysis by this method : — 



Weight of reduction-tube -f- precipitate 



" " precipitate 



Weight of reduction-tube after ignition 

 Loss by ignition 

 Weight of residue 



Weight of CaCl tube after ignition 

 " " " « before " 

 " " water 

 « " oxygen =.1175 — .0992 = 



Weight of crucible -|- CraOa 

 " " " + filter ash 



" Cr^Oa 

 " " soluble salts = .6537 — .1674 = 



In the compound whose formula is CrjOs CrOs, the ratio of the number 

 of atoms of oxygen to the number of atoms of chromium is that of 2 : 1. 

 By adding another equivalent of oxygen to the chromic oxide found, 

 we shall make the ratio of the atoms of oxygen to the atoms of chro- 

 mium that of the compound Cr.^Os CrOa. Hence the proportion. 



(Equiv. CrA) (Equiv. O) = (CrA found) (1 equiv. O) 

 76.48 : 8 =0.1674 : a; = 0.0175 



