GENERALIZATION. 251 



All gases expand by heat, according to the one same law, 

 and by nearly the same amount ; the specific heats of 

 equivalent weights are equal, and the densities, though 

 not the same, are exactly proportional to the atomic 

 weights. All such gases obey the general law, that the 

 volume multiplied by the pressure, and divided by the 

 absolute temperature, is constant or nearly so. The laws 

 of diffusion and transpiration are the same in all cases, 

 and, generally speaking, all physical laws, as distinguished 

 from chemical laws, which apply to one gas apply equally 

 to all other gases. Even when gases differ in chemical or 

 physical properties, the differences are minor in degree 

 or number. Thus the differences of viscosity are far less 

 marked than in the liquid and solid states. Nearly all 

 gases, again, are colourless, the exceptions being chlorine, 

 the vapours of iodine, bromine, and some other sub- 

 stances. 



Only in one single point, so far as I am aware, do gases 

 present distinguishing marks unknown, or nearly so, in 

 the solid and liquid states. I mean as regards the 

 light given off when incandescent. Each gas, when suf- 

 ficiently heated, yields its own peculiar series of rays, 

 arising from the free vibrations of the constituent parts 

 of the molecules when pursuing separate paths. Hence 

 the possibility of distinguishing gases by the spectro- 

 scope. But the molecules of solids and liquids appear 

 to be continually in conflict with each other, so that 

 only a confused noise of atoms is produced, instead of a 

 definite series of luminous chords. At the same tempera- 

 ture, accordingly, all solids and liquids give off nearly 

 the same rays when strongly heated, and we have in 

 this case a single exception to the general rule of the 

 greater generality of properties in gases. 



Liquids are in many ways intermediate in character 

 between gases and solids. While incapable of possessing 



