RAIN-MAKING. 479 



termine the energy of the molecular bombardment. By letting 

 tlie vibrating molecules of the solid or the gas come in contact 

 Avith the parts of our skin to which certain special sense nerves 

 are distributed, we feel the sensation of heat, and we are accus- 

 tomed to say that the expansion of the solid or the gas is due to 

 heat. The total measure of the energy which any mass of matter 

 has on account of the motion of its molecules is determined by 

 the amount of heat i. e., molecular motion which it must give to 

 other bodies before its molecules can come to rest. The higher 

 the temperature of the mass the more heat or the more molecular 

 motion it has. 



The atmosphere is, in general, made up of two different kinds 

 of molecules. These molecules are, of course, very small so 

 small that no possible magnifying power can ever bring them 

 into view. Their size is, in fact, so small as compared with the 

 length of a light-wave that no image of one could be produced by 

 reflected light. Still, there are several independent methods of 

 calculating their approximate size, and, since these different meth- 

 ods lead to fairly accordant results, we may assume that their 

 approximate size is known. According to Lord Kelvin's compu- 

 tation, if a drop of water were magnified to the size of the earth 

 its molecules would become larger than shot and smaller than 

 cricket balls, perhaps about the size of marbles. They are so 

 close together in the air that the number in a cubic inch is repre- 

 sented by the number ten raised to the twenty-third power. Be- 

 ing so close together, and being at the same time in raj^id motion, 

 they must have frequent collisions, and, according to Maxwell's 

 calculation, a molecule of air at ordinary temperatures would 

 have seven or eight hundred thousand millions of collisions in a 

 second of time. While these figures, both for size and number, 

 can convey no definite meaning to us, they may aid us in pictur- 

 ing to ourselves the tremendous agitation which is constantly 

 going on within our atmosphere or within the mass of any other 

 gas. 



Within the body of a liquid the conditions are similar, except 

 that here the molecules are so close together that they can not be 

 said to have any free path at all, and are, accordingly, in a state 

 of perpetual collision. They are not, as in a solid, held to any 

 definite position with reference to the surrounding molecules, but 

 are hindered by a force called cohesion or capillarity from escap- 

 ing from the liquid altogether. What the nature of this force is 

 is not known, but it is evidently a pressure of some kind exerted 

 upon the molecules tending to push them closer together. 



Notwithstanding this force, the molecules of the liquid are in 

 rapid vibration, and at the free surface of the liquid they are be- 

 ing continually bumped off by the molecules below them. When 



