150 PROGRESS OF SCIENCE IN THE CENTURY. 



dynamical theory of heat. '' The fundamental idea 

 that a gas was an assemblage of moving particles had 

 been put forward by D. Bernouilli and by Herepath, 

 and Joule had in 1851 made a great step in advance 

 by calculating the mean translational velocity of 

 these particles. . . This idea, in the hands of Kronig 

 and Clausius, gave birth to the modern kinetic theory 

 of gases, which has been so splendidly worked out 

 by Clausius and Maxwell, and since then perfected 

 in detail by Boltzmann, 0. E. Meyer, Van der Waals, 

 and many others/' * 



UNDULATORY THEORY OF LIGHT. 



The Emission Theory. — Throughout the eigh- 

 teenth century the corpuscular or emission theory of 

 light was almost universally accepted by physicists. 

 The theory was that all luminous bodies emit with 

 equal velocities inconceivably minute elastic corpus- 

 cles which travel at great speed in straight lines in all 

 directions. 



The Modern View. — l^owadays, however, it is the 

 unanimous view of those who are familiar with the 

 facts that light is not a material substance, but a 

 form of energy, or a mode of motion, in fact the re- 

 sult of ethereal waves. When a body gives forth 

 light, w^e no longer suppose that it emits corpuscles, 

 as a grain of musk does into the air ; we believe that 

 it sets agoing undulatory movements in the ether. 

 We believe furthermore that the phenomena of light 

 are essentially of the same nature as those of electro- 

 magnetic radiation. The contrast of the theories in 

 the two centuries is characteristic, and it is interest- 

 ing to enquire how the modern view was developed. 



* E. von Meyer, History of Chemistry, trans. 1891, p. 414. 



