CHAPTER XV 



THE MOLECULAR THEORY 



A. THE PROPERTIES OF GASES 



The atomic and molecular theories. The chief purpose 

 of Dalton's atomic theory was to explain the laws which 

 determine the composition of chemical compounds. The 

 molecular theory, which we owe to Avogadro, was on the 

 other hand very largely a physical theory, introduced in 

 order to account for the extremely simple properties which 

 characterise all substances when in the gaseous state. This 

 simplicity is specially obvious in the changes of volume which 

 are produced by 



(1) changes of pressure (Boyle, 1662) ; 



(2) changes of temperature (Charles, 1787); 



(3) chemical combination (Gay-Lussac, 1809). 



Boyle (1660) shows that air is an " elastic fluid." In 



his " New Experiments, Physico-Mechanical, touching the 

 Spring of the Air, and its Effects," published in 1660 

 (Works, 1725, II. 407-474), Boyle described an air-pump 

 (Fig. 45) which was an improvement upon the pump used by 

 Otto von Guericke in the celebrated experiment of the Magde- 

 burg hemispheres (1654). With the help of this pump Boyle 

 showed that a bladder, strongly tied at the neck, could be 

 dilated and even burst by the " spring " of the air which 



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