1040 Dynamic Theory. 



it unless it hits something ; but if it hits another body, as when the 

 earth hits a meteor, the amount of mass motion that is stopped 

 by the collision goes on as heat. But the heat is not all got out 

 of it till all the mass motion is stopped, and that means, when all 

 the separate molecules have ceased to move in whole or in part, and 

 have delivered their motions to their environing ether. The hotness of 

 the body continues in a decreasing degree during this delivery, and 

 ceases when it stops. So we see the hotness is not in the ponderable 

 particles but in the ether. A consideration of this idea will help us to 

 understand the remarkable fact stated on page 337, that the electric 

 arc does not heat. The heat arises when the motion of the mass, 

 whether it be that of a big one or only a molecule, is stopped or 

 diminished. According to the table of specific heats given at page 

 403, the atoms of all solid bodies without regard to their weight are 

 heated to the same degree of temperature by the same amount of heat. 

 The same is true in regard to gases. Equal volumes of all simple 

 gases contain the same number of atoms, and gases in equal volume 

 have the same specific heat. But equal weights of gases differ in the 

 nnmber of atoms they contain, and they differ inversely in the same 

 proportion in their specific heats as shown in the following table, in 

 which the specific heat of water is 1.00, and that of equal weights and 

 equal volumes of the other substances are placed opposite their names. 



Equal Equal Atomic Atomic 

 Weights Volumes Weights Heats 



Air 0.237 



Oxygen - - 0.218 0.240 16. 3.488 



Nitrogen - - 0.244 0.237 14. 3.416 



Hydrogen ... - 3.409 0.236 1. 3.409 



Chlorine - 0.121 0.296 35.6 4.295 



Bromine Vapor - - 0.055 0.304 80. 4.400 



In a true gas the particles are not supposed to have any attraction 

 for each other on account of their distances apart, and in oxygen, 

 nitrogen, and hydrogen, there is none of sensible amount. But bro- 

 mine is a vapor when above the temperature of 63; below that it is a 

 solid. Chlorine can be condensed to a liquid by a pressure of only 

 four atmospheres ; so in these two there appears to be still a degree of 

 cohesive attraction, which a part of their extra specific heat is required 

 to overcome. 



Now the explanation given of this uniformity of atomic heats is that 

 when a light atom has a certain amount of heat expended on it, it is 

 set to move off at a certain rate of speed, and if it were not stopped it 

 would continue its flight indefinitely in a straight line. When the same 

 amount of heat is expended on an atom weighing for example sixteen 

 times as much, it too will be started off on a straight flight, but at only 

 one-fourth the rate of speed ; the energy remaining the same, the 

 velocities are inversely in proportion to the square root of the masses 



