690 Proceedings of Royal Society of Edinburgh. [sess. 
even after they solidify, though they are greatly deficient in 
elasticity,* hut the permanent gases, oxygen, etc., retain their 
elasticity till almost all their electricity is gone, and that is almost 
to the zero of temperature. It is through this retention of their 
elasticity that I account for their maintaining the gaseous form 
through such a great range of temperature. 
For fear of being misunderstood, let us suppose two species of 
atoms of the same volume, hut one ten times the weight of the 
other. The lightest would have ten times the elasticity, for its 
particles would have ^/lO times the velocity of those of the other, 
and supposing them to start as perfect gases, at a very high tem- 
perature, the heavier would have cooled down far quicker than the 
other, for it would be the first to liquefy and solidify, and owing 
to the number of connections between the molecules in the liquid 
or solid state, emission of latent heat would proceed at a far more 
rapid rate. But this must be noted, that when the lighter had 
cooled down to the same temperature as the other (say to the zero 
temperature), it would have radiated precisely the same quantity 
of heat, for they both started with the same quantity of latent 
heat , and when they had passed through the same range of tem- 
perature they must have radiated the same quantity of heat away. 
11. Contact Electricity. 
It follows from the foregoing that at the surface of contact of 
a gas with a liquid, or solid, the state of electrification cannot 
be the same in both, for the gaseous molecules at such surfaces not 
meeting with moleculeshaving the same motion as they have them- 
selves, do not get the same amount of their electricity converted 
into heat as if they encountered molecules of their own kind ; they 
are therefore left with an altered amount of electricity at such 
surfaces, and which change in amount is known in the Science of 
Electricity as “contact electrification,” giving rise to “difference of 
* Elasticity . — I am inclined to think that there must be two conditions 
of elasticity, viz. , the elasticity of great particle-speed and small density, and 
the elasticity of great particle-density, or close-grainedness, as I have called 
it, and slow speed. I should say that the permanent gases possess the former 
and the metals the latter, other substances ranging between these. The 
former can be overcome by great pressure, but the latter only slightly, or 
not at all. 
