434 SCIENTIFIC THOUGHT. 



agree to date the real birth, not the incubation, of any 

 scientific idea from the moment when it was set forth 

 in definite figures, and with mathematical precision per- 

 mitting of a precise verification by actual test, the modern 

 theory of gases was born in Manchester in the school 

 34. of Dalton, when Joule in 1857 actually calculated the 



Joule's cal- ... . * 



cuiations. velocity with which a particle of hydrogen at ordinary 

 atmospheric pressure and temperature must be moving, 

 assuming that this 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. 

 Joule gave up the older and vague ideas of a rotatory or a 

 vibratory motion of the particles of a gas which had been 

 floating about since the time of Hooke l in- various theories, 

 and adopted the suggestion of Daniel Bernoulli, known to 

 him through Herapath, that all particles of gaseous matter 

 are in a natural state of rectilinear motion, which is 

 changed only by the encounter with other particles or by 

 the walls of the containing vessel on which they impinge, 

 and from which they rebound. 2 



Clausius, ' Die mechanische Warme- 

 theorie' (Braunschweig, 1889-91, 

 p. 2, &c.) See also 0. E. Mayer, 

 ' Die kinetische Theorie der Gase ' 

 (2d ed., Breslau, 1895, part i. p. 11). 



1 See Tait, ' Properties of Matter,' 

 2d ed., p. 289, also J. P. Joule's 

 Memoir on ' Heat and the Constitu- 

 tion of Elastic Fluids,' 1848, re- 

 printed in ' Scientific Papers,' vol. 

 i. p. 290, &c. 



- The real proof that the kinetic, 

 in contradistinction to what we may 

 call the Newtonian, view of the 

 motion of the molecules of a gas is 



the correct one, and that Newtonian 

 (attracting and repelling) forces 

 play only a subordinate, if any, part 

 in the observable phenomena of 

 gaseous bodies, is based upon Joule 

 and Thomson's experiments made 

 in 1853. It belongs to quite a 

 different line of reasoning, neither 

 chemical nor mechanical, but going 

 upon the principle introduced into 

 scientific thought about the middle 

 of the century, that heat and work 

 are convertible terms and equivalent 

 quantities. Now, it was generally 

 assumed, before Joule and Thomson 



