40S PROCEEDINGS OF THE AMERICAN ACADEMY. 



water by centrifugal settling, the silver chromate was dried at about 

 160° in an electric oven, and powdered in an agate mortar. 



Since in the case of Sample III the silver nitrate was added to 

 the chromate, while in preparing Sample IV precipitation took place 

 in the reverse fashion, a comparison of the two samples would not only 

 throw light upon the effect of hydrolysis, but also show whether the 

 occlusion of potassium chromate or silver nitrate was to be feared. 



The Analysis of Silver Chromate. 



The fact that salts dried by prolonged heating at 100°, or at even 

 higher temperatures, usually contain appreciable amounts of moisture, 

 owing to included mother liquor, is a point which has been over- 

 looked by most earlier investigators, 23 and the oversight throws doubt 

 on much otherwise very careful work. In exact work the residual 

 water must either be corrected for or entirely avoided. The simplest 

 fashion of drying a substance perfectly is to fuse it in a current of dry 

 gas. In the case of the silver chromate, however, this is not practi- 

 cable, for even at 300° incipient decomposition sets in. Upon attempt- 

 ing to dissolve in nitric acid samples dried in air at that temperature, 

 a slight insoluble residue was always obtained, while heating in a cur- 

 rent of oxygen gave no better results. Since the moisture cannot be 

 entirely expelled from silver chromate by heating at a moderate tem- 

 perature, it must be determined by the analysis of separate portions of 

 the substance which have been treated in some definite fashion. 



Experiments showed that at temperatures below 225° the salt was 

 not appreciably changed, hence this temperature was chosen as a suit- 

 able one at which to heat the salt preparatory to analysis. The silver 

 chromate was therefore always heated in a current of pure dry air for 

 two hours at 225°, in order to obtain the separate portions in as nearly 

 as possible the same condition. 



The drying apparatus was constructed entirely of glass, rubber con- 

 nections being especially avoided. A current of air was passed first 

 over red-hot copper oxide to destroy organic matter, then through 

 successive Emmerling washing towers. In the first were beads drenched 

 with silver nitrate solution, in the second with a strong solution of 

 potassic hydroxide containing much potassic manganate, and in the 

 last three with concentrated sulphuric acid. The already very dry air 

 was then passed through a long tube containing resublimed phosphoric 

 anhydride spread over a large surface of glass beads and ignited 



33 Richards, Proc. Am. Phil. Soc, 42, 28 (1903). 



