THE KINETIC THEORY OF MATTER 419 



but for the most joart the old ones remain. The atomic theory of matter, 

 for example, was not even tonched when radio-activity and the divisi- 

 bility of the atom were brought to light, for nobody who had gone 

 beyond the high school stage in science ever thought of asserting an 

 indivisible atom, and that simply because we had no basis for asserting 

 anything about the insides of the atom. We knew that there was a 

 smallest thing which took part in chemical reactions, and we named 

 that thing the atom, leaving its insides to the future, and the future 

 proved itself abundantly able to take care of the trust. 



Coming now to the first of our two theories, it is probably carrying 

 coals to Xewcastle to explain to an intelligent audience to-day what are 

 the essential elements of the kinetic theory of matter, but I will at least 

 carry enough of these coals to make a logical stepping-stone from the 

 familiar to the unfamiliar. 



The kinetic theory, then, when divorced from all non-essentials, is 

 merely the assertion that everything in this world of ours is in a state 

 of restless, ceaseless, seething motion, that all matter is composed of 

 minute parts called molecules which are eternally pounding and jostling 

 against one another. In gases these molecules are so far apart that the 

 forces of attraction which exist between them are quite negligible and 

 they dart hither and thither like gnats in a swarm, only with the stupen- 

 dous speed of a mile a second (in the case of hydrogen) and richochette 

 unceasingly against one another and the walls of the containing vessel, 

 producing by this bombardment all the familiar phenomena of pneu- 

 matic tires and gaseous bodies generally. If you could magnify the air 

 in an ordinary room just a thousand million times, that is, enough to 

 make a good-sized marrowfat pea swell to the size of the earth, yore 

 would see objects about as big as a football — we will not say of what 

 shape, because we do not know anything about it, but they would prob- 

 ably be of the same shape in a given gas — and if the motions would stop 

 long enough to enable us to get a snap shot of the whole situation, you 

 would see on the average one of these objects in a cubical space ten 

 feet on a side. Then if you let them go again you would see each of these 

 footballs shoot on the average through thirty such imaginary cubical 

 •rooms before it hit another. This distance we call the mean free path 

 of a gas molecule. 



In the liquid state the molecules are packed closely together by 

 cohesive forces, yet they continually wriggle and squirm over and 

 around one another, so that if you will be content this time with a 10 

 million-fold magnification, the liquid would look very much like a mass 

 of wriggling squirming maggots — not a pretty picture perhaps, but a 

 fairly accurate one I think. 



In solids the molecules are for the most part locked up tightly in 

 crystalline forms so that their motions are reduced to mere trembling. 



