WHAT THIS WORLD IS MADE OF 



every part. From this, and similar facts, it is possi- 

 ble to get an idea of the rate at which the particles 

 of the gas travel. So, too, the rate at which heat is 

 diffused supplies further information, for the theory 

 is that the heat is transmitted from one particle to 

 the other, and that to effect the transfer the mole- 

 cules must collide. A perfect vacuum is a perfect 

 insulator to ordinary heat. From this, the distance 

 traversed between the collisions may be computed. 

 Finally, it takes less power to run an electric fan, 

 say, in one kind of gas than in another. One 

 offers less friction than another. From these and 

 other observations the mathematician finds no 

 trouble in telling you the number of molecules in a 

 quart of air, how fast they fly, and how often they 

 smash into each other; and he will tell you, too, 

 that the molecules cannot be above nor below a cer- 

 tain size in order to account for the observed facts. 

 So one might go on. Ordinary air is a very bad 

 conductor of electricity. As you exhaust it, thin it, 

 it gets better and better, so that when you have 

 such a vacuum as in the little Crookes tube which 

 produces the Rontgen rays, it is a very good con- 

 ductor. There is a simple relation between the 

 number of molecules and the conductibility. This, 

 and the behavior of light also, agrees perfectly with 

 the values found in other ways. So, without going 

 any further, it will be seen that the measurement 



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