416 PROCEEDINGS OF THE AMERICAN ACADEMY. 



excess of acid accumulated in the solution during the precipitation of 

 the silver chrornate, so that the precipitate formed under distinctly acid 

 conditions, although the acidity was not sufficient to present any dan- 

 ger of the formation of dichromate. Samples III and IV, on the other 

 hand, since they were made from potassium chrornate, which is markedly 

 hydrolyzed, were formed under distinctly basic conditions, and the 

 precipitation or occlusion of basic salts is to be feared. Such occluded 

 basic salts would tend to raise the percentage of silver in the chro- 

 rnate. However, Sample IV yielded slightly higher results than 

 Sample III, while on account of the method of precipitation the reverse 

 is to be expected ; for Sample III was precipitated by adding the 

 silver nitrate to the chrornate, while Sample IV was precipitated by 

 adding the chrornate to the silver solution, the mother liquor remain- 

 ing neutral in both cases. Too much emphasis should not be laid upon 

 the slight apparent difference in the composition of the different sam- 

 ples of salt, since the variations in the experiments with the same 

 sample are as large as the differences between the samples. Hence the 

 average result from the different samples is employed in the final calcu- 

 lations, all the analyses being given equal weight in each series. 



In addition to the specimens of silver chrornate, the preparation and 

 analysis of which have been described, two other interesting specimens 

 * wei - e prepared. One was formed by adding a 0.04 normal silver nitrate 

 solution to a solution of chromic acid of similar concentration. On 

 account of the solubility of silver chrornate in nitric acid solutions, pre- 

 cipitation was only partial. The precipitate was washed and dried, and 

 upon analysis was found to contain so little silver that the presence of 

 a small proportion of dichromate was certain, a result which was hardly 

 to be expected in the light of Sherrill's experiments. 



The second sample was prepared by heating ammoniacal solutions 

 of silver chrornate in platinum vessels, the chrornate being gradually 

 precipitated as the ammonia was expelled. This material yielded 

 somewhat irregular results, which on the whole indicated too high per- 

 centages of silver, and hence the presence of basic salts, a result which 

 could have been predicted from a consideration of the conditions of 

 preparation. 



It is to be noted that Series I and Series II yield percentages of 

 silver differing by less than four thousandths of a per cent, a highly 

 satisfactory agreement, which indicates purity of the halogen acids em- 

 ployed as well as experimental accuracy. 



If the percentage of silver in silver chrornate is 65.0333, the molecular 

 weight of silver chrornate may be calculated from the atomic weight of 

 silver, and from the latter value the atomic weight of chromium by dif- 



