A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 183 



ADSORPTION OF AIR BY SILVER PHOSPHATE. 



Since the silver phosphate was in a very finely divided condition and since 

 many fine powders have the power of adsorbing appreciable quantities of air or 

 other gases, the possibility of the adsorption of air by silver phosphate was in- 

 vestigated. The method of experimenting and the apparatus were very similar 

 to that used by Baxter and Tilley for investigating the behavior of iodine 

 pentoxide: 



Two weighing-bottles were constructed with long, very well ground stoppers which ter- 

 minated in stopcocks through which the tubes could be exhausted. These tubes were very 

 closely of the same weight and of very nearly the same internal capacity. The tubes were first 

 exhausted and compared in weight by substitution. Next they were filled with dry air and 

 again weighed, the weighing being carried out with stopcocks open. Both steps were then 

 repeated with essentially the same results.* 



In these two experiments, when air was admitted, the counterpoise gained 

 0.00028 and 0.00021 gm. respectively (average 0.00025) more than the tube which 

 was later to contain the silver phosphate. After 22.69 g^i. of pure dry silver 

 phosphate had been placed in the tube, the tube and its counterpoise were ex- 

 hausted and the difference in weight determined. When dry air at 25° C. and 

 766 mm. was admitted to both the tube containing the silver phosphate and the 

 counterpoise, the counterpoise gained 0.00443 g^i. more than the tube. There- 

 fore, the air displaced by the silver phosphate was 0.00443 — 0.00025 — 

 0.00418 gm. Since 22.69 g^i. of silver phosphate of density 6.37 have a volume 

 of 3.56 c.c, the volume of pure air displaced at 25° C. and 766 mm. should 

 weigh 0.00425 gm.2 



The experiment was then repeated. After the air had been exhausted from the 

 tube and its counterpoise, the tube containing the silver phosphate was heated 

 gently. No gas was evolved. The tube and its counterpoise were then weighed 

 by substitution. When dry air at 24.5° and 767 mm. was admitted to both, the 

 counterpoise gained 0.00445 gm. more than the tube containing the silver 

 phosphate. Therefore the air displaced by the silver phosphate was 0.00445 — 

 0.00025 = 0.00420 gm., whereas the weight of air displaced, calculated from the 

 density of the salt, is 0.00426 gm. 



The agreement between the experimental results and those calculated from 

 the denstiy of silver phosphate on the assumption that no adsorption takes 

 place is close enough to show that no significant amount of adsorption occurs. 



1 Baxter and Tilley: Jour. Amer. Chem. Soc, 31, 214 (1909); Zeit. anorg. Chem., 61, 310. 

 (See page 130.) 



^ Rayleigh's value for the density of air at 0° and 760 mm., 1.293 gm. per liter, is used. 

 Proc. Roy. Soc, 53, 147. 



