BIOLOGICAL 



xxvii.] GENERALISATION. 601 



pending upon the directions of the axes of elasticity, which 

 we must not expect in uniform solids. Liquids, compared 

 even with non-crystalline solids, exhibit laws of far less 

 complexity and variety ; and gases assume, in many 

 respects, an aspect of nearly complete uniformity. To 

 trace out the branches of science in which varying degrees 

 of generality prevail, would be an inquiry of great interest 

 and importance ; but want of space, if there were no other 

 reason, would forbid me to attempt it, except in a very 

 slight manner. 



Gases, so far as they are really gaseous, not only have 

 exactly the same properties in all directions of space, but 

 one gas exactly resembles other gases in many qualities. 

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

 by nearly the same amount ; the specific heats of equiva- 

 lent weights are equal, and the densities are exactly pro- 

 portional 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, apply equally 

 to all gases. Even when gases differ in chemical or phy- 

 sical properties, the differences are minor in degree. Thus 

 the differences of viscosity are far less marked than in the 

 liquid and solid states. Nearly all gases, again, are colour- 

 less, the exceptions being chlorine, the vapours of iodine, 

 bromine, and a few other substances. 



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 sufficiently heated, 

 yields its own peculiar series of rays, arising from the free 

 vibrations of the constituent parts of the molecules. Hence 

 the possibility of distinguishing gases by the spectroscope. 

 But the molecules of solids and liquids appear to be con- 

 tinually 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 temperature, accordingly, 

 all solids and liquids give off nearly the same rays when 

 strongly heated, and we have in this case an exception to 

 the greater generality of properties in gases. 



