276 THE POPULAR SCIENCE MONTHLY. 



late Master of the Mint, upon the rate at which different gases were 

 mixed together. He found that if he divided a vessel by a thin parti- 

 tion made of black-lead or graphite, and put different gases on the 

 two opposite sides, they would mix together nearly as fast as though 

 there was nothing between them. The difference was, that the plate 

 of graphite made it more easy to measure the rate of mixture ; and 

 Dr. Graham made measurements and came to conclusions which are 

 exactly such as are required by the molecular tlieory. It is found 

 by a process of mathematical calculation that the rate of diffusion of 

 different gases depends upon the weight of the molecules. Now^, a 

 molecule of oxygen is sixteen times as heavy as a molecule of hydro- 

 gen, and it is found upon experiment that hydrogen goes through a 

 septum or wall of graphite four times as fast as oxygen does. Four 

 times four are sixteen. We express that rule in mathematics by say- 

 ing that the rate of diffusion of gas is inversely as the square root of 

 the mass of its molecules. If one molecule is thirty-six times as heavy 

 as another the molecule of chlorine is nearly that multiple of hydro- 

 gen it will diffuse itself at one-sixth of the rate. 



This rule is a deduction from the molecular theory, and it is found, 

 like innumerable other such deductions, to come right in practice. 

 But now observe what is the consequence of this. Suppose that, in- 

 stead of taking one gas and making it diffuse itself through a wall, 

 w^e take a mixture of two gases. Suppose we put oxygen and hydro- 

 gen into a vessel which has one side of it made of graphite, and we 

 exhaust the air from the other side, then the hydrogen will go through 

 this wall four times as fast as the oxygen will. Consequently, as 

 soon as one side is full there will be a great deal more hydrogen in it 

 than oxygen that is to say, that wg shall have sifted the oxygen 

 from the hydrogen, not completely, but in a great measure, precisely 

 as by means of a screen we can sift large coals from small ones. Now, 

 suppose, when we have oxygen gas unmixed with any othei*, the mole- 

 cules are of two sorts and of two different weights. Then you see 

 that if we make that gas pass through a porous wall, the lighter parti- 

 cles would pass through first, and we should get two different speci- 

 mens of oxygen gas, in one of which the molecules would be lighter 

 than in the other. The properties of one of these specimens of oxy- 

 gen gas would necessarily be different from those of the other, and 

 that difference might be found by very easy processes. If there were 

 any perceptible difference between the average weight of the mole- 

 cules on the two sides of the septum, there would be no difficulty in 

 finding that out. No such difference has ever been observed. If w^e 

 put any single gas into a vessel, and we filter it through a septum of 

 black-lead into another vessel, we find no difference between the gae 

 on one side of the wall and the gas on the other side. That is to say, 

 if there is any difference it is too small to be perceived by our present 

 means of observation. It is upon that sort of evidence that the state- 



