252 THE PRINCIPLES OF SCIENCE, 



different elasticity in different directions, and thus de- 

 nuded of the rich geometrical complexity of solids, they 

 retain the variety of density, colour, degrees of trans- 

 parency, great diversity in surface tension, viscosity, co- 

 efficients of expansion, compressibility, and many other 

 properties which we observe in solids, but not for the 

 most part in gases. Though our knowledge of the phy- 

 sical properties of liquids is thus much wanting in 

 generality at present, there is ground to hope that by 

 degrees laws connecting and explaining the varieties 

 of character may be traced out. Liquids ought to be 

 compared together, not at uniform temperatures, but at 

 points of temperature similarly related to the points of 

 fusion and ebullition. 



Solids are in every way contrasted to gases. Each solid 

 substance has its own peculiar density, hardness, com- 

 pressibility, degree of transparency, tenacity, elasticity, 

 power of conducting heat and electricity, magnetic pro- 

 perties, capability of producing frictional electricity, and 

 so forth. Even different specimens of the same kind of 

 substance will be widely different, according to the acci- 

 dental treatment it has received. And not only has 

 each substance its own specific properties, but, when 

 crystallized, its own properties peculiar to each direc- 

 tion, regard being had to the axes of crystallization. 

 The velocity of radiation, the rate of conduction of heat, 

 the coefficients of expansibility and compressibility, the 

 thermo-electric properties, all vary in different crystallo- 

 graphic directions. 



It is highly probable that many apparent differences 

 among liquids, and even among solids, will be resolved 

 and explained, when we learn to regard them under ex- 

 actly corresponding circumstances. The extreme gene- 

 rality of the properties of gases is really only true at an 

 infinitely high temperature, when they are all equally 



