MODEEN IDEAS ON THE END OF THE WOELD JAUMANN. 217 



may be said of the cooling of the sim, which should follow in ac- 

 cordance with the law of energy. It was supposed for a long time 

 to be self-evident that the clunate of the earth had grown constantly 

 cooler, but this idea has been entirely abandoned. Fluctuations less 

 than 10 degrees centigrade on both sides of the mean temperaXure 

 have often occurred, several tinites in Europe, thus placing these 

 regions now under tropical conditions, and now under the conditions 

 of the Arctic Zone. But from this point of view the most remote 

 ages of the geologic history of the earth differ not at all from the 

 present epoch. Glacial formations, extensive but not thick, have 

 been found in early Cambrian strata.^ At that time the tempera- 

 ture was not higher but lower than in our epoch, and more than a 

 hundred million years have passed since then. 



One can with difficulty admit of the existence in the sun of a 

 supply of energy able to endure without appreciable decrease, for so 

 long a time, the enormous expenditure due to radiation. The sta- 

 bility of the planetary system and the inexhaustible luminous power 

 of the sun are, furthermore, to a certain extent verified by direct 

 geologic observation. 



How is it that the law of gravitation and the principle of the 

 conservation of energ}^ fail so entirely in their prophecy concerning 

 the end of the world? What is the hidden defect of these laws 

 which, as the foundations of physics, have given such magnificent 

 results within narrower limits, and how can they be given an en- 

 tirely correct form ? 



Regarding these really fundamental questions of theoretical 

 physics, I feel myself called upon to speak, in so far as they fall 

 within the field of my own studies. I should point out, however, 

 that questions are concerned which are far from being decided, 

 and that I can treat here only their " phenomenalistic " aspect. The 

 Newtonian hypothesis of the attraction of a star on a remote body, 

 directly and instantly, without the physical intervention of an in- 

 termediary medium, was an abstraction nearly accurate, though at 

 bottom little true to nature. Laplace himself admitted the progres- 

 sive transmission of gravitation,- He supposed that this effect was 

 propagated, though at great speed, through the cosmic ether. The 

 magnetic forces between two magnets were likewise supposed at 

 first to act immediately at a distance. Faraday recognized eventually 

 that the air or similar medium contained between the two magnets 

 (the magnetic field), far from being indifferent, was in a state of 

 tension, and that the magnetic effects of one magnet on another were 

 propagated from point to point, from one particle to the particle 

 immediately adjoining it. It is thus that the elementary action 



'Compare, for example, Walther, History of the Earth, 1908, p. 199. 

 -Laplace Mecanique coleste, vol. 4, p. ?A7. 



