PHYSICAL SCIENCE IN THE NINETEENTH CENTURY 357 



KINETIC THEORY OF GASES. CLAUSIUS. The modern theory of 

 gases was born . . . when Joule in 1857 actually calculated the 

 velocity with which a particle of hydrogen . . . must be moving, 

 assuming that the atmospheric pressure is equilibrated by the 

 rectilinear motion and impact of the supposed particles of the gas on 

 each other and the walls of the containing vessel. This meant taking 

 the atomic view of matter in real earnest, not merely symbolically, 

 as chemists had done. M erz. 



The theory owes its full development, however, to the re- 

 searches of Maxwell, Clausius and Boltzmann. 



The great turning-point, indeed, lay in the kinetic theory of gases, 

 which . . . had introduced quite novel considerations by showing 

 how the dead pressure of gases and vapors could be explained on the 

 hypothesis of a very rapid but disorderly translational movement 

 of the smallest particles in every possible direction. 



THE CONCEPTION OF ENERGY. Newton's Principia contains 

 by implication the modern notion of energy but the first 

 clear and consistent fixing of the modern terminology is found 

 in Poncelet's Mecanique industrielle, 1829. The idea of work was 

 thus developed from the standpoint of the engineer notably 

 under the influence of Rankine ; while on the other hand, it is a 

 not less remarkable fact that Black, Young, Mayer and Helmholtz 

 all came to their scientific work through another form of applied 

 science medicine. 



A considerable step toward the general idea of the conservation 

 of energy was taken by Rumford in his determination of the 

 mechanical equivalent of heat, but the final achievement is due 

 mainly to Joule in England and Mayer and Helmholtz in Germany. 

 In 1847 Helmholtz read before the Physical Society of Berlin one 

 of the most remarkable papers of the century (Die Erhaltung 

 der Kraft), in which he says with full justice :- 



I think in the foregoing I have proved that the above mentioned 

 law does not go against any hitherto known facts of natural science, 

 but is supported by a large number of them in a striking manner. I 

 have tried to enumerate as completely as possible what consequences 

 result from the combination of other known laws of nature, and how 



