16 Transactions of the Society. 



» 



water, either in combination or absorbed by the solid ; and, finally, 

 cliromium compounds from the chromic acid used in precipitation. 

 The absence of chromium was shown by fusing the residue from 

 the experiments described below with sodium carbonate and 

 potassium nitrate, dissolving the mass in water, adding dilute 

 sulphuric acid and hydrogen peroxide, and shaking with ether. 

 No trace of blue colour was detected in the ether. 



The carefully weighed black substance was placed in a porce- 

 lain boat in a hard glass tube through which a current of carefully 

 purified liydrogen was passed. When all the air was displaced 

 from the apparatus the powder was heated. The water contained 

 in the substance was driven off, and was collected in a carefully 

 weighed phosphorus pentoxide tube. If the powder consists of 

 metallic osmium and water, the loss in weight of the l)oat and 

 substance, after cooling, should be exactly equal to the gain in 

 weight of the phosphorus pentoxide tube. If, however, the sub- 

 stance contains an oxide of osmium, the increase in weight of the 

 phosphorus pentoxide tube will be greater than the loss in weight 

 of the boat, since water will be formed from the oxygen contained 

 in the substance. The gain in weight of the tube will now be 

 made up of two parts : (1) The water originally contained in tlie 

 powder as such ; (2) the water formed from the oxygen in the 

 powder, the weight of which may be calculated from the loss in 

 weight of the ])Owder and the total weight of water formed. It 

 was found that more water was formed than corresponded with the 

 loss in weight of the powder, and the ratio of oxygen to osmium in 

 the latter was found to be 2 : 1. The material is therefore osmium 

 dioxide, OsOa, and not metallic osmium. Water was also present ; 

 but although the composition corresponded closely with the 

 formula OsOaoHaO — and a hydrated oxide has been described by 

 former experimenters (S) — the exact state of this water casnot at 

 present be decided with certainty. 



The experimental details were as follows : Hydrogen was 

 generated from a considerable quantity of pure electrolytic zinc 

 and hydrochloric acid (1 vol. of pure concentrated acid to 4 vols, 

 of water) in a bottle provided with a dropping funnel for the acid 

 and a run-off tubulure at the bottom. Admission of air was there- 

 fore reduced to a minimum. The gas was freed from acid spray 

 and vapour by bubbling slowly through caustic soda solution in a 

 wash bottle, and was roughly dried in a calcium chloride tower. 

 To remove all traces of oxygen from the gas it was next passed 

 through a heated tube containing platinized asbestos, and the 

 moisture w^as then completely removed by passing through a long 

 tube containing calcium chloride in the first part and phosphorus 

 pentoxide in the second part. The two materials were separated 

 by a column of glass wool. 



