176 RESEARCHES UPON ATOMIC WEIGHTS. 



DRYING OF SILVER PHOSPHATE. 



Unfortunately, owing to the high melting-point of silver phosphate, it was 

 not feasible to fuse the silver phosphate before its analysis in order completely 

 to eliminate all water. Instead it was heated in a platinum boat, in a current 

 of pure dry air, at a temperature of about 400° for 7 hours, and then by means 

 of the bottling apparatus (page 8) it was inclosed in its weighing-bottle with- 

 out coming in contact with t:he moist air of the laboratory. During this heating 

 the access of light to the sample was prevented. The continuous current of 

 air which passed over the silver phosphate during the heating was driven by a 

 water pump successively through an Emmerling tower containing beads mois- 

 tened with silver nitrate solution, through a tower containing small pieces of 

 fused caustic potash, then through three towers containing beads drenched 

 with concentrated sulphuric acid, and finally through a long tube containing 

 phosphorus pentoxide which had been resublimed in a current of air. The 

 hard-glass tube containing the platinum boat was surrounded by blocks of 

 aluininum (page 78) which were jacketed with asbestos on the top and sides 

 and heated directly from below by a large burner. The platinum boat was not 

 attacked in the least, as was shown by the fact that its weight remained con- 

 stant. 



It was feared that in spite of this prolonged heating the silver phosphate still 

 retained a trace of water, but by making the conditions in the different experi- 

 ments as nearly uniform as possible it was hoped that the amount of water 

 retained would be constant. Proof will be given later that the drying was 

 higlily efficient. 



The salt thus prepared for analysis was allowed to stand over night in a 

 desiccator covered with a black cloth in the balance room, and was then weighed 

 in its glass-stoppered bottle by substitution, with the use of another weighing- 

 bottle of very similar surface and volume as a counterpoise. 



The balance was a nearly new No. 10 Troemner balance. It was easily sen- 

 sitive to 0.02 mg. The weights had already been used in an investigation of 

 the atomic weight of sulphur,^ and were restandardized with a very gratifying 

 result. None of the corrections found differed by as much as 0.02 mg. from those 

 found a year before, and only a few by o.oi mg. The balance was provided with 

 a few milligrams of radium bromide of radio-activity loooo to dispel electri- 

 cal charges generated during the handling of the weighing-bottles with cork- 

 tipped pincers. 



1 Richards and Jones: Pub. Car. Inst., No. 69, 69 (1907); Jour. Amer. Chem. Soc, 29, 826; 

 Zeit. anorg. Chem., 55, 72. 



