10 BEYOND THE LIMITS OF VISION. 



heavier than hydrogen, move with only about half that velocity. 

 This is one element of the problem. 



Again, if different gases are placed in communication with 

 each other, they instantly commence mixing together. ~No mat- 

 ter how quiet they may be, or how different in weight within 

 certain limits, after a certain time they are found to be thoroughly 

 commingled. This is accomplished by the incessant knocking of 

 particles against particles until they are finally scattered equally 

 in every direction. Now it is by careful and repeated experiments 

 on the rate of diffusion of the different gases with each other 

 that physicists have calculated how many times in a second these 

 particles must be hitting against each other to produce the observed 

 results. It is found that the number of collisions which one atom 

 of hydrogen must make in a second is on the average 17,700 

 millions. Thus we have the distance an atom travels in a second, 

 and the number of hits it makes. From these figures we can very 

 easily tell the average distance the atom travels between each two 

 successive collisions, called the mean free path, and which must be 

 about the actual distance apart of these atoms. You see we are 

 beginning to get a little hold on these intangible and inscrutable 

 things. 



From the general principles of molecular mechanics there has 

 been deduced the following simple proportion : As a given 

 volume of any gas is to the amount of solid matter in it, so is 

 the average mean path of any one of its molecules to one-eighth 

 of the actual diameter of that molecule. Now a cubic foot of 

 steam, which is water gas, when condensed makes almost exactly 

 a cubic inch of water, which may be taken as very nearly the 

 solid matter in that amount of steam. Therefore, the cubic 

 inches in a cubic foot (1728) is to one, as the mean free path 

 (about one-millionth of an inch), is to one-eighth of the diameter 

 of a molecule of water vapor. The working out of this sum 

 gives the size of the ultimate particle of any gas as about the 

 250 millionth of an inch. Here we have the problem solved ; 

 not perhaps with a perfectly accurate result, but certainly some- 

 thing near it. We have got at the approximate dimensions of 

 the last unit of matter and it is an exceedingly small thing 



