CHAPTER I. 



INTRODUCTORY CHAPTER. 



THE KINETIC THEORY CONCEPTION OF MATTER. 



1. THE dynamical theory of gases, as developed in the present book, 

 rests solely upon what is commonly termed the Kinetic Theory of matter. 



The essential feature of the Kinetic Theory is that it interprets heat 

 in matter as a manifestation of a motion of the molecules which compose 

 the matter. It need hardly be said that this identification of heat and 

 motion is only a hypothesis : it never has been, and from the nature of 

 things never can be, proved. At the same time this hypothesis shews an 

 ability to explain and even to predict natural phenomena, such that there 

 can be little doubt that it rests upon a foundation of truth. 



2. The Kinetic Theory assumes at the outset that all matter, whether 

 solid, liquid or gaseous, is composed of a great number of small molecules, 

 and that these molecules are similar to one another so long as they are 

 molecules of the same chemical substance. In making this assumption 

 we are again relying upon an unproved and improvable hypothesis. This 

 second hypothesis is of course not peculiar to the Kinetic Theory but is 

 borrowed from the science of Chemistry. Chemistry further supposes 

 molecules to be capable of division into smaller bodies, namely atoms, and 

 modern investigations shew that these again may be capable of division 

 into yet smaller bodies ions, electrons, or corpuscles. The Kinetic Theory, 

 however, is concerned primarily with molecules only, and it is hardly 

 necessary, at any rate for the development of the pure Kinetic Theory, to 

 discuss any hypotheses or speculations as to the structure or divisibility 

 of molecules. 



Having once postulated this divisibility of matter into molecules, our 



every-day experience gives a certain amount of guidance as to the properties 



which are to be attributed to molecules. For -instance, we know that two 



bodies cannot occupy the same space; any attempt to compel them to do 



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