174 LECTURES TO SCIENCE TEA GHEES. 



from the centre ; so also all the phenomena observed and 

 grouped by Newton are included in the statement of the 

 law of force as he denned force to be. 



But there is no other theory in physics so firmly 

 demonstrated as this. Our belief in the truth of the 

 urdulatory theory rests on the fact that it explains a 

 vast number of totally different phenomena in a perfectly 

 clear way ; in fact, there are no phenomena in the theory of 

 light which, if they have not been explained by the 

 undulatory theory, we are not justified in saying could be 

 explained by it if our mathematical analysis were suf- 

 ficiently powerful to translate the meaning of the theory 

 completely. But the undulatory theory of light has 

 acquired certainty from a vast number of independent 

 sources as well. In order that the radiations from a 

 luminous body may be propagated through the ether, it is 

 necessary that the luminous body must have its molecules 

 in a state of rapid vibration, and not only has the whole 

 science of spectrum analysis led up to this, to point out 

 that the molecules of different bodies are vibrating in 

 certain definite periods, such as will give vibrations off to 

 the ether, and ultimately send them so as to strike the eye, 

 but also a number of theories which have been reached 

 independently, all tend and converge to this same point, 

 and show us that not only are the molecules of a body in 

 a state of rapid vibration, but that there is an ether 

 capable of transmitting those vibrations. The splendid 

 experimental results obtained by Professor Graham in 

 connection with the diffusion of gases have proved almost 

 beyond a doubt that the molecules of gases are moved about 

 with a very great velocity, and means have been devised 

 actually to measure the very velocity with which they are 

 moving. The researches of Clausius, Clerk Maxwell, 

 Thomson, Eankine, and others, have reduced the science of 

 the molecular motion of gases to an absolute certainty, so 

 far as theory can be certain, and they have explained tho 

 laws of gases perfectly upon this assumption. The very 

 size of one of these molecules* has been determined by Sir 

 William Thomson in four different manners, all of which 

 agree, within fair limits, with each other ; and from 

 arguments which it is impossible for me to go into at 

 present, Sir William Thomson has shown what is in all pro- 



