APPLICATION OF THE MOLECULAR HYPOTHESIS 89 



thought to mean the property of a given mass of gas in virtue of 

 which another gas can mix with the given mass. This property 

 is not diffusibility but permeability. It is the other gas, which 

 makes its way into the given gas, which is diffusing. Diffusion 

 is spontaneous motion of the parts of a gas away from their original 

 location. Unless this motion is into an empty space, the diffusing 

 molecules must, of course, move into another body of gas. In 

 the case of the jars of hydrogen and air (p. 55), each gas moved in 

 part out of its original jar (diffused), and each received parts of 

 the other gas into its jar (was permeated). 



Boyle's Law and Charles' Law. Passing now to Boyle's 

 law (p. 45), the thing to be accounted for is that when a sample of 

 a gas diminishes in volume, its pressure increases in the same pro- 

 portion. Let the diagram (Fig. 37) represent a cylinder with a 

 movable piston, upon which weights may be placed to resist the 

 pressure. Now the pressure exercised by the gas under the 

 piston cannot be like the pressure of the hand upon a table, since 

 we have just assumed that the particles are not even approxi- 

 mately at rest, and the spaces between them are enormous com- 

 pared with the size of the molecules themselves. The gaseous 

 pressure must therefore be attributed to the colossal 

 hailstorm which their innumerable impacts upon the 

 piston produce. If this is the case, the compressing 

 of a gas must consist simply in moving the partition 

 downwards, so that the particles as they fly about 

 are gradually restricted to a smaller and smaller space. 

 Their paths become on an average shorter and 

 shorter. Their impacts upon the walls become more 

 and more frequent. So the pressure which this causes 

 becomes greater and greater, and is proportional to 

 the degree of crowding (the concentration) of the molecules. 



There are two other points to be added. When we diminish the 

 volume to one-half, we find from experience that the pressure 



i 



FIG. 37 



