602 THE PRINCIPLES OF SCIENCE. [CHAP. 



Liquids are in many ways intermediate in character 

 between gases and solids. While incapable of possessing 

 different elasticity in different directions, and thus denuded 

 of the rich geometrical complexity of solids, they retain the 

 variety of density, colour degrees of transparency great 

 diversity in surface tension, viscosity, coefficients of expan- 

 sion, compressibility, and many other properties which we 

 observe in solids, but not for the most part in gases. 

 Though our knowledge of the physical properties of liquids 

 is much wanting in generality at present, there is ground 

 to hope that by degrees laws connecting and explaining the 

 variations may be traced out. 



Solids are in every way contrasted to gases. Each solid 

 substance has its own peculiar degree of density, hardness, 

 compressibility, transparency, tenacity, elasticity, power 

 of conducting heat and electricity, magnetic properties, 

 capability of producing frictional electricity, and so forth. 

 Even different specimens of the same kind of substance will 

 differ widely, according to the accidental treatment received. 

 And not only has each substance its own specific properties, 

 but, when crystallised, its properties vary in each direction 

 with regard to the axes of crystallisation. The velocity of 

 radiation, the rate of conduction of heat, the coefficients of 

 expansibility and compressibility, the thermo-electric pro- 

 perties, all vary in different crystallographic directions. 



It is probable that many apparent differences between 

 liquids, and even between solids, will be explained when 

 we learn to regard them under exactly corresponding 

 circumstances. The extreme generality of the properties 

 of gases is in reality only true at an infinitely high tempe- 

 rature, when they are all equally remote from their con- 

 densing points. Now, it is found that if we compare 

 liquids for instance, different kinds of alcohols not 

 at equal temperatures, but at points equally distant 

 from their respective boiling points, the laws and co- 

 efficients of expansion are nearly equal. The vapour- 

 tensions of liquids also are more nearly equal, when com- 

 pared at corresponding points, and the boiling-points 

 appear in many cases to be simply related to the chemical 

 composition. No doubt the progress of investigation will 

 enable us to discover generality, where at present we only 

 see variety and puzzling complexity. 



