1 FOUNDATIONS OF THE HYPOTHESIS 5 



* 



the motion which we call heat forms which necessarily 

 differ with the nature of the body considered, its state of 

 aggregation and other qualities we have succeeded in 

 showing that a whole series of important laws of nature 

 necessarily follow from these assumptions, and we may 

 therefore be sure that we have discovered the mechanical 

 cause of these laws in the circumstances of the ultimate 

 particles of bodies. Specially successful have been the 

 labours of those who sought to explain the nature of the 

 gaseous state doubtless because the heat-motion in gases 

 obeys the simplest laws. From a very simple assumption 

 as to the nature of this motion we have deduced, not only 

 all the laws already known for gaseous bodies, but also new 

 properties which have been most beautifully verified by 

 experiment. 



There has thus arisen from the joint labours of the 

 workers in this field a special theory of the gaseous state 

 which was formerly known as the dynamical, 1 but is now 

 better called the kinetic, 2 theory of gases. In this work I 

 have endeavoured to collect and arrange the scattered con- 

 tributions of individual authors that have appeared in 1 

 periodicals of all kinds. 



2. Hypotheses with, regard to Heat-motion 



The ultimate elements of bodies whose motion we wish 

 to investigate are not freely movable each by itself; they 

 are bound together by mutual forces their affinity, whence / 

 arise combinations of atoms into larger masses called mole- 

 cules. 



We may therefore distinguish two kinds of heat-motion, 

 atomic and molecular. By the latter we understand the 

 translatory motion of the centroid of the atoms that form 

 the molecule, while as atomic motion we count all the 



1 Maxwell, Phil. Mag. [4] xix. 1860, p. 19, xx. 1860, p. 21. 



2 So far as I know, this name was first used by Lord Kelvin (Sir W. 

 Thomson) in an address before the British Association at Edinburgh (B.A. 

 Rep. 1871, p. 93) ; Maxwell afterwards adopted it (Nature, viii. 1873, p. 298 ; 

 Scientific Papers, ii. p. 343). 



