24 RESEARCHES UPON ATOMIC WEIGHTS. 



nitrogen and hydrobromic-acid gases was thoroughly dried, first by pure fused 

 calcium bromide, in the tube U, and then by resubhmed phosphorus pentoxide 

 in the tube W. 



If desired the nitrogen could be passed directly through the phosphorus pent- 

 oxide tube into the bottling apparatus XY, in which the fusion took place. 



Air was purified and dried by passing over fused potassium hydroxide in 

 the tower A, concentrated sulphuric acid in the towers B and C, and phosphorus 

 pentoxide in the tubes D and E. 



The cadmium bromide, contained in a weighed platinum boat, was heated 

 gently in a current of nitrogen until a small quantity of residual crystal water 

 was expelled, then strongly in a current of nitrogen and hydrobromic acid until 

 fused. After the salt had cooled, the hydrobromic acid was displaced by nitro- 

 gen and this in turn by dry air. During the displacement of the hydrobromic 

 acid by nitrogen and air a sHght backward current of gas was maintained 

 through the tube W and the trap V. The boat was then transferred to the 

 weighing-bottle in which it was originally weighed, and the stopper was inserted 

 without an instant's exposure of the salt to moisture, by means of the bottling 

 apparatus which has been referred to on page 9. The weighing-bottle was 

 then allowed to stand in a desiccator near the balance case for some time before 

 it was weighed. 



METHOD OF ANALYSIS. 



Next the boat was transfered to a flask and the salt was dissolved in about 

 300 c.c. of the purest water. The weighing-bottle was rinsed and the rinsings 

 were added to the solution. Then the solution was filtered into the glass-stop- 

 pered precipitating flask through a tiny filter to collect a trace of insoluble 

 matter, and the filter-paper and residue were ignited at a low temperature in a 

 weighed porcelain crucible. This residue, which usually amounted to less than 

 0.1 mg. and was never as much as 0.2 mg., did not contain detectable quantities 

 of cadmium, and probably consisted of silica and a trace of platinum re- 

 moved from the boat during the fusion, for the boat, when reweighed, in most 

 cases was found to have lost a few hundredths of a milligram. No change in 

 weight could be found when the boat was first dried and weighed, then ignited 

 and reweighed. The difference between the weight of the residue and the loss 

 in weight of the boat was subtracted from the weight of the cadmium 

 bromide. 



From the corrected weight of the cadmium bromide very nearly the requisite 

 quantity of pure silver was calculated. This silver was weighed out and dis- 

 solved, in nitric acid diluted with an equal volume of water, in the flask de- 

 scribed on page 12. After the silver was dissolved, the solution was diluted to 

 twice its volume and was heated until free from nitrous fumes. Then it was still 

 further diluted until not stronger than i per cent, and was slowly added, with 

 constant stirring, to the i per cent solution of cadmium bromide in the precipi- 



