STRUCTURE OF AIR: MOLECULAR THEORY 51 



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

 to distribute itself more or less evenly through the room with- 

 out 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 careful 

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

 time, each one of them is soon evenly distributed 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 ? 



47. Some questions regarding gases. In the last section we 

 learned certain facts which were not explained and which 

 ought to give rise to many questions if we are interested in 

 those facts. In the first place, how can one substance pass 

 through another substance, as when illuminating gas diffuses 

 through the air from one part of the space to another part ? 

 It seems unlikely that the air and the illuminating gas really 

 occupy the same space at the same time. 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 the substance of the sponge is not really compressed. 

 Such a substance as iron does not appear to be porous, and 

 we do not find it easy to compress iron into smaller space. 

 May it be true that the air is really porous? If so, that might 

 also explain how the illuminating gas passed through the air 

 and how it is possible for air and other gases to expand. 



