HERMANN VON HELMHOLTZ 289 



stars which are governed by exactly the same laws as those subsist- 

 ing between the earth and moon ; that, therefore, the light and heat 

 of terrestrial bodies do not in any way differ essentially from those 

 of the sun, or of the most distant fixed star ; that the meteoric stones 

 which sometimes fall frorn external space upon the earth are com- 

 posed of exactly the same simple chemical substances as those with 

 which we are acquainted. We need, therefore, feel no scruple in 

 granting that general laws to which all terrestrial natural processes 

 are subject, are also valid for other bodies than the earth. We will, 

 therefore, make use of our law to glance over the household of the 

 universe with respect to the store of force, capable of action, which 

 it possesses. 



A number of singular peculiarities in the structure of our planetary 

 system indicate that it was once a connected mass with a uniform mo- 

 tion of rotation. Without such an assumption, it is impossible to 

 explain why all the planets move in the same direction round the sun, 

 why they all rotate in the same direction round their axes, why the 

 planes of their orbits, and those of their satellites and rings all nearly 

 coincide, why all their orbits differ but little from circles ; and much 

 besides. From these remaining indications of a former state, astron- 

 omers have shaped an hypothesis regarding the formation of our plane- 

 tary system, which, although from the nature of the case it must 

 ever remain an hypothesis, still in its special traits is so well sup- 

 ported by analogy, that it certainly deserves our attention. It was 

 Kant who, feeling great interest in the physical description of the 

 earth and the planetary system, undertook the labour of studying the 

 works of Newton, and as an evidence of the depth to which he had 

 penetrated into the fundamental ideas of Newton, seized the notion 

 that the same attractive force of all ponderable matter which now 

 supports the motion of the planets, must also aforetime have been 

 able to form from matter loosely scattered in space the planetary 

 system. Afterwards, and independent of Kant, Laplace, the great 

 author of the Mecaniqiic Celeste, laid hold of the same thought, and 

 introduced it among astronomers. 



The commencement of our planetary system, including the sun, 

 must, according to this, be regarded as an immense nebulous mass 

 which filled the portion of space which is now occupied by our system, 

 far beyond the limits of Neptune, our most distant planet. Even 



