60 MORRIS LOEB 



independent bodies moving in a very definite way and having 

 very definite size, is found in a well-known experiment known 

 as osmose. Suppose that in this room there was a partition 

 which was pierced by a number of narrow doors, and on the 

 one side were a number of grown persons and on the other an 

 equal number of children, and the order was given that they 

 should distribute themselves equally over the total space. 

 The children, moving more quickly, and having no difficulty 

 in squeezing through the narrow doors, would soon get over 

 to the other side, while the grown persons were still struggling 

 to get through. The result would be that at first the actual 

 number of people on that side of the room where the children 

 had first been would be less than the number on the other 

 side. Now, instead of persons, I take light and heavy mole- 

 cules, the lightest molecules we know being hydrogen, and the 

 molecules constituting the gases of the air being over fourteen 

 times as heavy. In order to imitate the partition wall, I take 

 this tube made of porous porcelain; the openings are so fine 

 that we may well imagine the gas molecules having some trouble 

 in getting through. If I fill the inside of this tube with air, 

 and now fill this jar outside with hydrogen, the hydrogen will 

 get in very much faster than the air can get out, and the result 

 will be that we have a larger number of particles or molecules 

 per cubic inch inside than there were originally; they have 

 to find more room for these, and you see how they obtain it, 

 by pressing this liquid out in a sort of fountain. If I reverse 

 the experiment, I obtain the opposite effect. There is a rather 

 ingenious practical application of this principle which is per- 

 haps worth showing. Instead of holding water to be pressed 

 out, this tube contains mercury; as the liquid metal is pressed 

 down it will rise on the opposite side and make an electric 

 contact, which causes this bell to ring. Instead of taking 

 hydrogen I shall this time take illuminating gas, and here 



