THE ASTRONOMICAL VIEW OF NATUKE. 379 



principle of universal gravitation. This is, in my opinion, 

 one of the greatest proofs of the truth of this admirable 

 principle. As to this principle, is it a primordial law 

 of nature ? Is it only a general effect of an unknown 

 cause ? Here the ignorance in which we are as to the 

 ultimate properties of matter stops us, and removes all 

 hope that we shall ever be able to answer these questions 

 in a satisfactory manner." 



In the meantime, as I have tried to show, the clue 

 afforded by this principle has led physicists by strict 

 analysis, by observation, by cleverly arranged experi- 

 ments as well as by guesses drawn from analogy, to the 

 discovery of many unknown phenomena, to the fixing in 

 mathematical language of interesting relations, and in 

 general to a large extension of the field of natural know- 

 ledge. No wonder that a principle which has done, and 

 is still doing, such valuable service in physical astronomy 

 should have done much to establish the astronomical 

 view of nature. 1 As one of the latest representatives of 

 physical science abroad has said, " The present generation 



1 This view was concisely put by 

 Poisson at a time when the corpus- 

 cular theory of the imponderables 

 light, heat, and electricity still 

 reigned supreme in the Continental 

 school : " Toutes les parties de la 

 maticre sont soumises a deux sortes 

 d'actions mutuelles. L'une est at- 

 tractive, independante de la nature 

 des corps, proportiounelledu produit 

 des masses, et en raison inverse du 

 carre" des distances : elle sVtend 

 iudofiniment dans 1'espace, et pro- 

 duit la pesanteur universelle et 

 tous les phenomenes d'equilibre et 

 du mouveruent qui sont du ressort 

 de la me'canique celeste. L'autre 



est attractive et repulsive ; elle 

 de'pend de la nature des particules 

 et de leur quantite" de chaleur ; 

 son intensite ddcroit tres rapide- 

 ment quand la distance augmente, 

 et devient insensible, des que la 

 distance a acquis une , grandeur 

 sensible" ('Journal del'Ecole poly- 

 technique,' cahier xx, p. 4, 1831). 

 See also Clerk Maxwell, ' On the 

 Equilibrium of Elastic Solids' (1850, 

 reprinted in ' Scientific Papers,' vol. 

 i. p. 30), where a similar assumption 

 is stated as the basis of the mathe- 

 matical theories of Navier, Poisson, 

 Lame", and Clapeyrou. 



