xxvii.] GENERALISATION. 603 



Tn some cases substances exhibit the same physical pro- 

 perties in the liquid as in the solid stnte. Lead has a high 

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

 crystallised 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 other 

 ])]iysical changes. Colour, absorptive power for heat or 

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

 same in liquids and gases. Iodine and bromine possess a 

 deep colour whenever they are chemically uncombined. 

 Nevertheless, we can seldom argue safely from the pro- 

 perties of a substance in one condition to those in another 

 condition. Ice is an insulator, water a conductor of 

 electricity, and the same contrast exists in most other 

 substances. The conducting power of a liquid for elec- 

 tricity increases with the temperature, while that of a solid 

 decreases. By degrees we may learn to distinguish 

 Ijetween 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 pro])erties 

 of a substance with respect to light seem generally to 

 depend upon the molecule ; thus, the power of certain 

 substances to cause the plane of polarisation of a ray of 

 light to rotate, is exactly the same wliatever be its degree 

 of density, or the diluteness of the solution in which it is 

 contained. Taken as a whole, the j>hysical properties of 

 substances and their quantitative laws, present a problem 

 of infinite complexity, and centuries nnist elapse before any 

 moderately complete generalisations on the subject become 

 possible. 



Uniform Properties of* all Matter. 



Some laws are held to be true of all matter in the 

 universe absolutely, without exception, no instance to the 

 contrary having ever been noticed. This is the case with 

 the laws of motion, as laid down by Galileo and Newton. 

 It is also conspicuously true of the law of universal gravi- 

 tation. The ri.se of modern physical science nifiy perhap.s 

 be considered as beginning at the time when Galileo 



