300 GREAT MEN OF SCIENCE 



and examined. Maxwell added the idea of electric waves, 

 which should emanate from such oscillations, and showed 

 what the properties of these waves should be, and Hertz, 

 somewhat later, was actually able to discover and study these 

 waves. All these discoveries we shall still have to consider 

 in their chronological order. 



In order to form a true estimate of Clausius and Thomson, 

 but particularly of the former, we must now once more turn 

 to the phenomena of heat. 



Clausius was not only (from 1850 onwards) the founder of 

 thermodynamics, but also (from 1857 on) developed the 

 kinetic theory of gases. While the former starts from the 

 two laws already mentioned, according to which heat is 

 treated as a form of energy without further enquiry into its 

 nature, the latter discusses the motion of the molecules, 

 which is the essential nature of heat; here heat actually 

 appears as that which Rumford already supposed it to be, 

 namely motion. Knowledge of nature was now sufficiently 

 advanced for this to be asserted with certainty, and also for 

 the nature of the molecular motion to be stated. This could 

 be done most completely for the gaseous state of matter. 

 Joule had already (1851) made a well-founded beginning of 

 this, which was no longer a simple and arbitrary hypothesis, 

 but was based upon his studies on the mechanical equiva- 

 lent of heat.^ 



Clausius now calculated, in a manner free from all objec- 

 tion, the velocities of the molecules and later their free paths, 

 that is to say, the distance through which they move in a 

 straight line between collisions. The velocities are very 

 great, the free paths very small at ordinary temperature and 

 pressure, so that the collisions of the molecules are very fre- 

 quent. These collisions take place according to the laws 

 already founded by Huygens for perfectly elastic bodies. 



1 Joule's Collected Paper;:, Vol. i, p; 290: 'Some remarks on heat and 

 the constitution of elastic fluids.' 



