APPLICATION OF THE MOLECULAR HYPOTHESIS 



93 



Summary. We may now summarize the principal facts 

 about gases in mass, with the corresponding features of the molec- 

 ular relations. 



Facts About Gases in Mass 



Corresponding Relations of Molecules 



Compressibility 



Diffusibility 



Permeability 



Non-settling 



Homogeneity 



Pressure 



Boyle's law 



Charles' law. 



Gay-Lussac's law. 

 Law of diffusion. . 



Gases can be liquefied, 

 and show lower com- 

 pressibility at high 

 pressures. 



Vacuum + molecules widely separated. 



Molecules in rapid motior. 



Empty space relatively large. 



Molecules perfectly elastic. 



Molecules of any one substance closely alike. 



Due to impacts of molecules. 



Pressure proportional to concentration of the 



molecules. Molecules move in straight lines 



and, when widely scattered, show no tendency 



to cohesion. 

 Rise in temperature increases the velocity of 



the molecules. 

 Avogadro's law. 

 Speed of molecules inversely proportional to 



square root of mass. 

 Molecules do possess some tendency to cohesion, 



and do occupy an appreciable fraction of the 



space when pressure is very great. 



The Properties of Liquids. The fact that even great pres- 

 sures produce little diminution in the volume of a liquid shows 

 that the free space, so predominant in gases, is relatively unim- 

 portant in liquids. This is only natural in view of the smaller 

 volume occupied by substances in the liquid state. Thus we 

 have seen (p. 62) that steam at 100 occupies nearly 1600 times 

 the volume of an equal weight of water. To reduce the volume 

 of water to four- fifths requires, not raising the pressure to five- 

 fourths as in the case of steam, but increasing it from 1 to 10,000 

 atmospheres. Evidently the molecules are already very closely 

 packed. 



This close packing of the molecules causes cohesion to be much 

 more pronounced in liquids than in gases. A small amount 

 of liquid, when poured into a vessel, does not fill the whole of 



