396 Structure of the Universe 



of crack, a gap or fissure in the uniform medium. It may be inter- 

 esting here to mention Kelvin's vortex atomic theory of matter, from 

 which so much was expected, but which has failed to give any clue 

 to gravity, and any theory of matter which does not contain a solu- 

 tion of the problem of gravitation can have no permanent value, for 

 gravitation is the supreme problem before physics to-day. The next 

 great advance in physical science lies in the solution of this problem. 



The Vortex Atomic Theory. 



Kelvin conceived a perfect fluid continuously filling space, and he 

 supposed that what we call an atom of matter is the rotating portion 

 of this fluid. We can make air, water or any other fluid more or 

 less rigid by imparting rapid motion to it. The motion differentiates 

 that portion of the fluid which is in motion from that portion which 

 is not in motion, like the smoke ring which sometimes ascends from 

 the funnel of a locomotive. The idea was that if the fluid were 

 frictionless and vortex motion once started in it that motion would 

 continue forever. He conceived that atoms might be composed of 

 such rings of ether in motion, the ether being supposed to be the 

 perfect fluid. The atoms were a sort of ether squirts. This theory 

 of matter was quite interesting and something new at the time, and 

 the dynamics of the theory were worked out by Helmholtz and J. J. 

 Thomson, but notwithstanding that it promised so much, it has been 

 for the most part given up, and it has not been able to throw any 

 light on the problem of gravitation. I spoke about the electron 

 theory of matter in a previous part of this lecture. Neither has this 

 theory, however fascinating in many respects, been able to produce an 

 explanation of gravity. The gravitational force is entirely different 

 from and belongs to another order than the electrostatic and electro- 

 magnetic forces, and all the endeavors to get* an explanation of gravi- 

 tation out of them ha^e led to negative results. 



Mass. 



We have spoken about mass. Now let us see what mass is. Of 

 course, mass has been defined as the amount of matter in a body, or 

 the inertia of a body. We should not confound mass with weight. 

 The book which is lying on this table has a certain weight here 

 which can be ascertained exactly, but if I transferred it to the Equator 

 of the earth it would weigh less than it does here, and if I took it to 

 the North Pole it would weigh more. In other words, the weight of 

 a body on the earth's surface depends on its distance from the cen- 

 ter of the earth, and vice versa, and we know that the surface of 

 the earth at the Equator is 13 miles further away from the center of 

 the earth than the North Pole is. But the book at any place on the 

 earth's surface would still have exactly the same mass, and indeed 

 it would still have the same mass at any place in the universe. 

 Weight depends on the force of gravity, and we know the force of 

 gravity varies at different points of the earth's surface, according to 

 their distance from the earth's center; but the mass does not vary 

 so long as the law of the conservation of matter holds good. If the 

 book were placed millions of miles from the earth away out in 



