504 Prof. Helmholtz on the Interaction of Natural Forces. 



planetary system, indicate that it was once a connected mass 

 with a uniform motion 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 direc- 

 tion 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 astronomers have 

 shaped an hypothesis regarding the formation of our planetary 

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

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

 supported by analogy, that it certainly deserves our attention, 

 and the more so, as this notion in our own home, and within 

 the walls of this town*, first found utterance. It was Kant 

 who, feeling great interest in the physical description of the 

 earth and the planetary system, undertook the labour of study- 

 ing 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 Mecanique 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 nebu- 

 lous mass which filled the portion of space which is now occu- 

 pied by our system, far beyond the limits of Neptune, our most 

 distant planet. Even now we perhaps see similar masses in the 

 distant regions of the firmament, as patches of nebulae, and 

 nebulous stars; within our system also, comets, the zodiacal 

 light, the corona of the sun during a total eclipse, exhibit rem- 

 nants of a nebulous substance, which is so thin that the light of 

 the stars passes through it unenfeebled and unrefracted. If we 

 calculate the density of the mass of our planetary system, accord- 

 ing to the above assumption, for the time wheu it was a nebu- 

 lous sphere, which reached to the path of the outmost planet, 

 we should find that it would require several cubic miles of such 

 matter to weigh a single grain. 



The general attractive force of all matter must, however, impel 

 these masses to approach each other, and to condense, so that 

 the nebulous sphere became incessantly smaller, by which, ac- 

 cording to mechanical laws, a motion of rotation originally slow, 

 and the existence of which must be assumed, would gradually 

 become quicker and quicker. By the centrifugal force which 

 * Konigsberg. 



