G. F. Becker — Kant as a Natural Philosopher. 109 



similar to those which the elder Herschel regarded as probable 

 in the earlier part of his career, and in which Laplace believed 

 prior to 1824. The subject has been under discussion ever 

 since and is not definitely settled. Of late years the applica- 

 tion of the spectroscope and the researches of Proctor have 

 made it seem possible that only one stellar system is visible. 

 If others exist they may be too far off for their light to reach 

 the earth, or have progressed too far towards extinction to be 

 sensibly luminous.* 



After Kant's explanation of the heat of the sun as due to 

 compression, the theory does not seem to have been revived 

 until Helmholtz enunciated it in 1854. f It now has the 

 adherence of Lord Kelvin, and, so far as I know, of all physicists 

 as the main source of solar radiation. 



The novelty of Kant's view of Saturn's rings was that he 

 supposed them to be composed of fume, or minute discrete 

 particles, instead of satellites, as Cassini and Wright had done. 

 It is rather strange that Laplace, though, like Kant, appealing 

 to Saturn as an illustration of the nebular hypothesis, should 

 yet have regarded the rings as solid. The older view, practi- 

 cally in the Kantian form, was reviewed by Maxwell, \ who was 

 evidently unaware of the earlier hypothesis. 



Little advance has been made in explanation of the Zodiacal 

 light since Kant's time. It is regarded as partly gaseous and 

 partly composed of solid matter, perhaps meteoroids. Its 

 spectrum seems to be that of reflected light. 



Kant's idea of a transition from planets to comets has not 

 received verification, and the orbit of Neptune is at present 

 the most nearly circular of all the planetary orbits, excepting 

 that of Venus, instead of the most eccentric as Kant would 

 have supposed. Hence his prediction that planets exterior to 

 Saturn would be found cannot be placed to his credit, its truth 

 being accidental. 



Kant was not the first to notice the decrease in density of 

 the planets Earth, Mars, Jupiter and Saturn, while the densi- 

 ties of Mercury and Venus were unknown in his time. Such 



*The possibility that ether envelopes a stellar system like an atmosphere with- 

 out extending indefinitely Into space, is perhaps worth considering in reference to 

 the hypothesis of the existence of distant star groups. 



f Helmholtz's paper. Ueber die Wechselwirkung der Naturkrafte, was a popular 

 lecture delivered in Konigsberg and, according to Thomson and Tait, was on the 

 occasion of a commemoration of Kaut (Nat. Phil., vol. ii. p. 49:5). In this paper 

 appeared also his views on the tidal retardation of the earth and the moon, and 

 the collapse of the solar system. Kant is mentioned as author of the nebular 

 hypothesis, but not in connection with these more detailed questions. It is cer- 

 tainly most curious that at a Kant celebration Helmholtz should have announced 

 the rediscovery of four of Kant's theories. 



^Proceedings R. S. Edin., vol. iv, 1858, and Adams prize essay, for 1856. Both 

 may be found in his collected works. 



