18(3 THE PRINCIPLES OF SCIENCE. 



whole planetary system through space must occasion a 

 similar displacement of at most 5". The ordinary aber- 

 ration can be readily detected with modern astronomical 

 instruments, because it goes through a yearly change in 

 direction or amount, but the systematic aberration is 

 constant and permanent so long as the planetary system 

 moves uniformly in a sensibly straight line. Only then 

 in the course of ages, when the curvature of the sun's path 

 becomes apparent, can We hope to verify the existence of 

 this kind of aberration. A curious effect also must be 

 produced by the sun's proper motion upon the apparent 

 periods of revolution of the binary stars. 



To my mind, some of the most interesting truths in 

 the whole range of science are those which have not been, 

 and in many cases probably can never be, verified by trial. 

 Thus the chemist assigns, with a very high degree of 

 probability the vapour densities of such elements as 

 carbon and silicon, which have never been observed sepa- 

 rately in a state of vapour. The chemist also is familiar 

 with the vapour densities of elements at temperatures at 

 which the elements in question never have been, and 

 probably never can be, submitted to experiment in the 

 form of vapour. 



Joule and others have calculated the actual velocity of 

 the molecules of a gas, and even the number of collisions 

 which must take place per second during their constant 

 circulation. Sir W. Thomson has not yet given us the 

 exact absolute magnitudes of the particles of matter, but 

 he has ascertained by several distinct methods the limits 

 within which their magnitudes must lie. Many of such 

 scientific results must for ever be beyond the power of 

 verification by the senses. I have elsewhere had occasion 

 to remark that waves of light, the intimate processes of 

 electrical changes, the properties of the ether which is 

 the base of all phenomena, are necessarily determined 



