170 PROGRESS OF SCIENCE IN THE CENTURY. 



is susceptible of strain. All energy appertains either 

 to matter or to ether, and is continually passing from 

 one to the other." * 



It is now time to turn to the actual progress of 

 scientific discovery and to note a few of the steps 

 which have led towards the modern views of matter, 

 as above suggested. 



A. In Connection with the Kinetic Theory of 

 Gases. In his Hydrodynamica (1738), Daniel 

 Bernouilli supposed a gas to consist of moving parti- 

 cles, and argued that the pressure, if due to the im- 

 pacts of these, must be proportional to the square of 

 their velocity. 



In 1816 (published 1821), Herapath followed on 

 the same tack, and in spite of fundamental errors 

 (e.g., that the temperature of a gas is measured by 

 the momentum of each of its particles), gave a 

 theoretical justification of Boyle's law (that with con- 

 stant temperature the product of pressure and volume 

 is constant). 



In 1846, Waterston (whose work was overlooked 

 until disinterred from the archives of the Royal So- 

 ciety of London by Lord Rayleigh in 1892) showed 

 that the temperature of a gas " is measured by the 

 mean kinetic energy of a single molecule, and that 

 in a mixture of gases the mean kinetic energy of 

 each molecule is the same for each gas," f thereby 

 furnishing the theoretical basis for the laws of Boyle, 

 Gay-Lussac, and Avogadro. 



In 1848, Joule used Herapath's results as a basis 

 for calculating the mean velocity of the molecules of 

 a gas, and obtained from hydrogen at freezing point 

 and atmospheric pressure the value of 6,055 feet 



* Loc. tit., pp. 499-500. 

 fGlazebrook. James Clerk Maxwell, 1896, pp. 118-19. 



