MATTER FORMED OF SMALL PARTICLES 111 



the pressure upon the surface, boiling will commence at a 

 correspondingly lower temperature. 



In the same manner as above it may be shown that the 

 saturated vapours of other liquids exert a final pressure, which 

 is proportional to the temperature, and that the temperature 

 at which any liquid boils is the temperature at which its 

 saturated vapour exerts a pressure equal to that upon the free 

 surface of the liquid. 



74. The Enunciation of Avogadro's Theory. The observa- 

 tion that most gases change their volume in almost exactly the 

 same degree, when they undergo the same changes of tempe- 

 rature or pressure, led to the theory of Avogadro. Accord- 

 ing to this theory, similarity of behaviour is caused by 

 similarity of structure. We must regard a given volume of a 

 gas as a system of very small invisible particles, separated 

 from each other by a distance which diminishes when the 

 pressure increases or the temperature falls, and grows larger 

 when the pressure decreases or the temperature rises. Dif- 

 ferent kinds of gases will have different kinds of particles ; 

 but equal volumes of all gases, at the same temperature and 

 pressure, will contain the same number of particles. 



The theory does not extend to the composition or structure 

 of the particles, or to other changes consequent on a change of 

 temperature. With these it is not concerned. It aims merely at 

 explaining the observation that gases undergo the same change 

 in volume when equally changed in temperature and pressure. 

 Further investigations are necessary before the accuracy or 

 limitation of the theory can be tested. It has the undoubted 

 advantage of simplicity. It offers no precise explanation of 

 the manner in which changes of volume are produced, nor of 

 the exact condition of the system at any given moment. It 

 assumes that a given change is effected by an average change 

 in the distance between the particles. Some may be wider 

 apart and some closer together than the average. 



Since nothing like any heterogeneity of structure, much 

 less isolation of particles, can ever be detected, it is indis- 

 pensable to the theory that the particles must be considered 

 as extremely small. 



