280 LECTURES TO SCIENCE TEACHERS. 



improved the process by admitting the nitric oxide very 

 slowly in minute bubbles into the volume of air with 

 continual agitation, and in that way he succeeded in 

 obtaining results very accordant indeed, but which were 

 not capable of giving complete analyses of the air, because 

 he also found, as has been recently found within the last 

 few months, that the nitric oxide prepared by the action 

 of acids on metals varies very considerably in composition. 

 Scheele used for the analysis of air anoiAer substance 

 altogether, namely, a mixture of iron filings and sulphur 

 made into a paste with water. This mix^o was placed 

 in a retort with the neck standing in water, and after 

 several hours, or sometimes several days, the diminution 

 of the volume of the gases or of the air was very percep- 

 tible. This is due to the absorption of oxygen by the 

 iron filings and sulphur, but it was soon shown that after 

 a time there is a formation of some sulphuric acid, which, 

 acting on the iron, liberates hydrogen, and therefore 

 there is ultimately a mixture of nitrogen and hydrogen 

 left in the retort which gives a greater volume than the 

 pure nitrogen which the air originally contained. This 

 process of Scheele's had been previously tried by Dr. Hales. 

 I tried an experiment of this kind during last week, and I 

 find it takes three or four days with a small quantity of 

 iron filings and sulphur, to absorb the oxygen from a 

 moderately large retort. This process is, therefore, far 

 too slow. Scheele also used a very ingenious mode of 

 analysing the air, but one which is not capable of any 

 direct application. He took a small bottle and placed it 

 in some zinc and sulphuric acid so as to generate hydrogen. 

 From the top of the bottle passed a narrow glass tube 

 drawn out to a jet, at which the hydrogen was inflamed. 

 This vessel was placed in a pneumatic trough, and a large 

 glass was inverted over it as rapidly as possible. Of 

 course the flask contained its own volume of atmospheric 

 air. The neck of the flask was placed in the water of the 

 trough, and of course, the hydrogen combining with the 

 oxygen produced a diminution of volume. But he noticed 

 the diminution until the moment the hydrogen w T as extin- 

 guished from the complete exhaustion of the oxygen, and 

 then measured the minimum volume. Of course after that 

 there began to be an increase of volume on the mixture of 



