STRUCTURE OF AIR MOLECULAR THEORY 59 



57. Diffusion of gases. If there is a leaking gas pipe in a 

 closed room, the odor of gas can soon be noticed in any part 

 of the room. The fact that the odor can be found in all 

 parts of the room indicates that the gas has spread to the 

 most remote corners. How did it get there ? If there were 

 currents of air, it might be carried by the currents, but the 

 same result is observed even if the air is quiet. It seems, 

 therefore, that the small amount of gas that has been released 

 from the pipe in one part of the room has somehow managed 

 to distribute itself more or less evenly through the whole 

 room without being much affected by the presence of the 

 air. In other words, although the room was occupied by 

 the gas which we know as the air, the gas which escaped 

 from the pipe has spread itself through the room as if it 

 were the only gas present. 



If other gases are used, the same results follow ; and care- 

 ful tests show that when several gases are in a space at the 

 same time, each one of them soon becomes evenly distrib- 

 uted throughout the space. This property that gases have 

 of mingling with one another when in contact, even if not 

 stirred, is called diffusion. The same property is shown in 

 liquids also. 



From these experiments it might appear that two or more 

 substances (gases in this case) can occupy the same space 

 at the same time. Do you think that this is really true ? 

 Is it true of solids ? of liquids ? 



58. Some questions regarding gases. How can one sub- 

 stance pass through another substance, as when illuminating 

 gas diffuses through the air from one part of the space to 

 another part? Certainly two persons or two books cannot 

 occupy the same space at the same time. Furthermore, how 

 may a substance be made to occupy less space, as it does 

 when air is compressed ? When a sponge, which is porous, 

 is compressed, the pores are made smaller, but it is the pores 

 which are really compressed, and not the substance of the 



