148 PROGRESS OF SCIENCE IN THE CENTURY. 



It seems that the clearness of the Newtonian view 

 of the movements of the heavenly bodies often sug- 

 gested to chemists and others who thought about 

 atoms and molecules, that these might be bound to- 

 gether in a manner comparable to a planetary system. 

 But the behaviour of gases and the phenomena of 

 heat (so long regarded as a substance) made it nec- 

 essary to suppose that forces of repulsion as well as 

 attraction existed between particles. Gradually the 

 intrusion of what Merz calls ^' the astronomical 

 view of nature " to support the incipient " atomic 

 view of matter " was found unavailing. The atomic 

 view passed from its static to its kinetic phase, and 

 we may particularly associate this important step 

 with the names of Joule, Clausius, and Clerk Max- 

 well. 



Although Bernouilli (1738), Herepath, Waterston 

 and many others must find their recognition in 

 learned histories, it was Joule who first gave precise 

 expression to the theory that all particles of gases may 

 be thought of as being in a natural state of rectilinear 

 motion, changed only by their mutual encounters, or 

 by their impinging on containing barriers. It was 

 soon after the half century (published 1857) that 

 Joule, as we have noted, calculated the velocity of a 

 particle of hydrogen at ordinary atmospheric press- 

 ure and temperature. The calculation presupposed 

 the previous discovery by Rumford, Davy, Mayer, 

 and Joule that heat is not a substance but a mode 

 of motion, and the experimental proof by Joule and 

 Thomson (1853) that in a gas allowed to expand 

 without doing work there is a very slight cooling, 

 due to the energy used up in overcoming the attract- 

 ing forces of cohesion. 



The general argument is simply that if heat can 



