250 Domestic Science 



11. Describe how you would arrange an experiment to demon- 

 strate that the products of combustion of ordinary coal-gas weigh 

 more than the coal-gas burnt. 



12. Cooking-salt frequently becomes very damp when exposed 

 to the air, while prepared table-salt under similar circumstances 

 remains dry. Suggest reasons for this difference in behaviour. 



13. One cubic centimetre of steam weighs '000805 g., and the 

 same volume of air weighs '00129 g. Calculate the weight of water- 

 vapour in 100 litres of air, if the air contains 1'5 per cent, by volume 

 of water-vapour. 



CHAPTER XV 



OXYGEN 



169. Nitrogen may be obtained from the air in a 

 state of comparative purity by any of the several means 

 described in Chapter xm. No simple method has been 

 devised whereby the oxygen may be directly separated 

 in similar fashion. It is possible to show indirectly 

 that this gas is actually present in the atmosphere 

 by carrying out somewhat lengthy and troublesome 

 experiments. The best known of these, ascribed to 

 the famous French scientist, Lavoisier, and used by 

 him to show that the ideas prevalent in his day con- 

 cerning the nature of the changes involved in rusting 

 and burning were entirely fallacious, deserves descrip- 

 tion. 



In the bulb of the curiously shaped retort shown in 

 the adjacent figure, Lavoisier poured a quantity of 

 mercury, which had been carefully weighed. The end 

 of the bent neck of the retort dipped under the surface 

 of mercury in the trough, and the air in the retort and 



