548 THE PRINCIPLES OF SCIENCE. [CHAP. 



present age of the world. As the earth's motion round the 

 sun combined with the motion of light causes the stars to 

 deviate apparently from their true positions to the extent 

 of about 1 8" at the most, so the motion of the whole plane- 

 tary system through space must occasion a similar displace- 

 ment of at most 5". The ordinary aberration can be readily 

 detected with modern astronomical instruments, because it 

 goes through a yearly change in direction or amount ; but 

 systematic aberration is constant 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 must also 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 i-ange of science are those which have not been, and 

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

 Thus the chemist assigns, with a very high degree of pro- 

 bability, the vapour densities of such elements as carbon 

 and silicon, which have never been observed separately in 

 a state of vapour. The chemist is also 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. Physicists have not yet given us the exact 

 magnitudes of the particles of matter, but they have ascer- 

 tained by several methods the limits within which their 

 magnitudes must lie. Such scientific results must be for 

 ever 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 neces- 

 sarily determined in a hypothetical, but not therefore a 

 less certain manner. 



Though only two of the metals, gold and silver, have 

 ever been" observed to be transparent, we know on the 

 ^rounds of theory that they are all more or less so ; we 

 can even estimate by theory their refractive indices, and 



