f; EXE HA L IZA TIOX. 253 



remote from their condensing 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 coefficients of expansion are nearly equal. The 

 vapour-tensions of liquids also are much more nearly 

 equal, when thus compared at corresponding points, 

 and the boiling-points themselves appear to be simply 

 related to the chemical composition in many cases. Xo 

 doubt the progress of investigation will often enable us 

 to discover generality, where we at present only see 

 variety and puzzling complexity. 



In some cases substances exhibit the same physical pro- 

 perties in the liquid as in the solid state. Lead has a 

 high refractive power, whether in solution, or in solid salts, 

 crystallized, or vitreous. The magnetic power of iron is 

 conspicuous, whatever be its chemical condition ; indeed, 

 the magnetic properties of substances, though varying 

 with temperature, seem not to be greatly affected by 

 physical changes. Colour, absorptive power for heat or 

 light rays, and a few other properties are also often the 

 same both in liquids and gases. Iodine and bromine 

 possess a deep colour whenever they are chemically un- 

 combined. Nevertheless, we can seldom argue safely 

 from the properties of a substance in one condition to that 

 in another condition. Ice is an insulator, water a con- 

 ductor of electricity, and the same contrast exists in most 

 other substances. The conducting power of a liquid for 

 electricity increases with the temperature, while that of a 

 solid decreases. By degrees we may learn to distinguish 

 between those properties of matter which depend upon 

 the intimate construction of the chemical molecule, and 

 those which depend upon the contact, conflict, mutual 

 attraction, or other relations of distinct molecules. The 

 properties of a substance with respect to light seem gene- 



