Philosophical State of the Physical Sciences. 503 



infinite space or what happens there; but the existence of the 

 aether, where our experience can follow it, is a physical reality. 

 The source of the motion which the aether acquires is not the sun; 

 for the most efficient cause of solar radiation is gravitation and 

 condensation. Our author avoids the vicious circle of making- 

 gravitation first the reason, and afterwards the consequence of 

 the motion of the aether. He says : — t( It follows that the sun's 

 heat, and the heat of masses in general, are stable quantities, 

 oscillating, it may be, like the planetary motions, about mean 

 values, but never permanently changing, so long as the Up- 

 holder of the universe conserves the force of the aether and the 

 qualities of the atoms. There is no law of destructibility ; but 

 the same Will that conserves can in a moment destroy." The 

 following remarks upon this theory deserve our attention: — "The 

 explanation of any action between distant bodies by means of a 

 clearly conceivable process, going on in the intervening medium, 

 is an achievement of the highest scientific value. Of all such 

 actions that of gravitation is the most universal and the most 

 mysterious. Whatever theory of the constitution of bodies holds 

 out a prospect of the ultimate explanation of the process by 

 which gravitation is effected, men of science will be found ready 

 to devote the whole remainder of their lives to the development 

 of that theory." 



The hypotheses of Challis and Le Sage have one thing in 

 common; the motion of the aether and the driving storm of atoms 

 must come from outside the world of stars. (i On either theory 

 the universe is not even temporarily automatic, but must be fed 

 from moment to moment by an agency external to itself." Our 

 science is not a finality. The material order which we are said 

 to know makes heavy drafts upon an older or remoter one, and 

 that again upon a third. The world, as science looks at it, is 

 not self-sustaining. We may abandon the hope of explaining- 

 gravitation, and make attraction itself the primordial cause. Our 

 refuge then is in the sun. When we qualify the conservation of 

 energy by the dissipation of energy, the last of which is as much 

 an induction of science as the first, the material fabric which we 

 have constructed still demands outward support. Thomson cal- 

 culates that, within the historical period, the sun has emitted 

 hundreds of times as much mechanical energy as is contained in 

 the united motions of all the planets. This energy, he says, is 

 dissipated more and more widely through endless space, and 

 never has been, probably never can be, restored to the sun 

 without acts as much beyond the scope of human intelligence 

 as a creation or annihilation of energy, or of matter itself, 

 would be. 



From the earliest dawn of intellectual life a general theory of 



