148 



RESEARCHES UPON ATOMIC WEIGHTS. 



was provided with ground glass stopcocks lubricated with Ramsay desiccator 

 grease. The silver chromate was gradually heated until fusion took place, and 

 a slow current of air was allowed to pass through the system for one half hour 

 in order to make certain that all moisture was carried into the absorption tube. 

 Finally the phosphorus pentoxide tube was reweighed. 



The pentoxide tube was weighed by substitution with the use of a counter- 

 poise of the same size and weight. Before being weighed both tubes were care- 

 fully wiped with a damp cloth and were allowed to stand near the balance case 

 for 30 minutes. Care was taken to equaUze the pressure inside and outside the 

 tubes by opening one stopcock immediately before hanging on the balance. 



In order to test the efficiency of the drying apparatus, blank experiments were 

 carried out by allowing a slow current of air to pass through the apparatus into 

 the weighed pentoxide tube. The variations in the weight of the tube were 

 never much larger than the probable error in weighing the tube. 



As is to be expected, the water content gradually decreases with increasing 

 temperature of heating. The extreme variation with specimens of silver 

 chromate which have been heated at 225° amounts to only 0.005 P^r cent. Evi- 

 dently the percentage of residual water is as constant as can be reasonably 

 expected, and the mean can safely be assumed to represent with sufficient 

 exactness the average proportion of water in the salt. Hence from every 

 apparent gram of silver chromate 0.000144 gm. is subtracted. 



