356 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The exact molecular weight of a compound of known 

 composition may be calculated in a similar way from its 

 approximate vapour-density and the exactly-known atomic 

 weights of its constituents. 



SUMMARY AND SUPPLEMENT. 



A. THE PROPERTIES OF GASES. 



Boyle, in 1660, with the help of a new air-pump, showed that 

 air was an "elastic fluid" which could be expanded to at least 

 152 times its original volume by merely diminishing the pressure 

 of the atmosphere upon it. To explain the " spring of the air" 

 he suggested that air was composed of elastic fibres, or (following 

 Descartes) of elastic spheres in rapid motion under the influence 

 of heat. In 1662, he showed that the pressure which air could 

 exert was inversely proportional to the volume which it occupied 

 (Boyle's Law). 



Mayow. in 1674, showed that air in which a candle or lamp 

 had expired, and "air" prepared by the action of acids on 

 metals, were just as elastic as common air and could be expanded 

 to an equal extent. 



Gay-Lussac, in 1802 (following Charles, 1787), showed that 

 air, oxygen, nitrogen and hydrogen all expand to theextent of 

 37-50 per cent, between o and 100 C. : nitric oxide, carbon 

 dioxide, sulphur dioxide, ammonia, hydrogen chloride and the 

 vapour of ether were also shown to expand at the same rate as 

 common air. Gay-Lussac therefore concluded that the volumes 

 of all gases and vapours are influenced to the same extent by 

 changes of temperature and of pressure. 



B. THE COMBINING VOLUMES OF GASES. 



Gay-Lussac and Humboldt, in 1805, confirmed the obser- 

 vations of Cavendish (1781) that two volumes of hydrogen unite 

 with one volume of oxygen to form water ; their experiments 

 gave the ratio 199^89 : 100. 



Gay-Lussac, in 1809, concluded that "gases always combine 

 in the simplest proportions when they act on one another." 



