32 i COMSTOCK— THE MODERN THEORY [-Mmi 22, 



of the general vibratory motion which (Hstinguishes a hot body from 

 a cold one. 



The electron, too, is now well within what we might call " the 

 exclusive circle of the truly real." This minute charged body has 

 by manv researches, among them the recent one of ^Slillikan, been 

 shown to be a definite reality, present in all matter and entirely or 

 largely responsible for all the phenomena we know as electrical. 



The other fundamental entity, energy, is also, if I may be allowed 

 the phrase, " mysteriously real." Radiant energy leaves the sun 

 eight minutes before it reaches the earth, and must, therefore, exist 

 during that time in the space between, dissociated from ordinary 

 matter. When it finally strikes some object and is absorbed, it gives 

 the object a thrust — that is, communicates momentum to it, as a 

 bullet would do — and at the same time, of course, it increases the 

 total energy of the body. In the same way, a body radiating energy 

 recoils during the emission in a way similar to a gun. 



All this is remarkably like the action of ordinary matter. We 

 can, however, say even more. There is very good reason for be- 

 lieving that, were it possible to shut up a large amount of radiant 

 energy in a hollow box, the inner surfaces of which were perfectly 

 reflecting, so that the rays would be reflected back and forth indefi- 

 nitely, we would find this confined energy to possess both mass and 

 weight. Not many decades ago such an idea would have seemed 

 absurd, l)ut it is hard now to avoid the conclusion that such would 

 be the case. 



I can do no better now than to descril)e in a few words the pic- 

 ture which we have today of the ultimate structure of matter. A 

 piece of matter is composed of particles called atoms, which, by 

 uniting in groups in various ways, form the characteristic aggre- 

 gates which we know as molecules. There are about one hundred 

 different kinds of atoms, varving in relative weiglits from one to 240, 

 and in relative volumes from one to about 16. The approximate 

 diameter of an atom is one one-hundred-millionth of a centimeter. 



These atoms are in ceaseless motion to and fro, the energy of 

 this motion determining what we call the temperature of the body. 

 Within the atoms and in the spaces between them are large numbers 

 of very much smaller particles known as electrons. They each 



