ATOMS AND MOLECULES 25 



under law, that they ask not an infinitesimal part of a 

 hair's-breadth more, and will not take an infinitesimal part 

 of a hair's-breadth less, and so the measure of increase, 

 which their mighty numbers claim, varies not. 



The volume of gases is acted on in the same measure 

 by pressure. It is inversely proportional to the pressure 

 to which they are subjected. Increase the pressure to 

 double the amount, and the volume is reduced to one- 

 half j to fourfold the amount, and the volume becomes a 

 fourth. Reduce the pressure from four pounds to two, 

 and the volume is doubled; to one, and it is enlarged 

 fourfold. 



The expansion and contraction of gases, according to 

 the measures of heat or pressure brought to bear upon 

 them, are thus the same for all substances in the gaseous 

 form. They all act in the same manner. They have 

 their motions and forces and condition ordered, measured, 

 and balanced so as to obey the same laws. There are 

 none inert, disobedient, or rebellious among them. They 

 are all active, all in perpetual motion, all exerting and 

 yielding to force, and alike related to the physical forces 

 which can be brought to bear on them to determine their 

 relative positions, all responding to their action with the 

 same readiness. 



Diffusion of gases. The diffusive power of gases is of 

 vital importance. When they do not enter into combina- 

 tion by contact, they pass through each other, and become 

 a mixture. However much they differ in specific gravity, 

 and though the heaviest be lowest, it will ascend, and the 

 lightest will descend. Fill a jar with hydrogen, and if 

 the temperature and pressure continue the same, there 

 cannot be put into it more than the right and definite 

 number of molecules. The same number of oxygen 

 molecules may, however, be added, and also of other 



