402 Mr. A. S. Davis on a Theory of Nebula and Comets. 



chemical union may only be able to exist as a gas so long as it 

 remains at a great heat. After the compound has formed, it will 

 cool down, and may ultimately assume the liquid or solid state. 



When liquid or solid matter is formed, it will gradually ag- 

 gregate and gravitate towards the centre of the nebula, and will 

 form into a nucleus there. It is probable that chemical action 

 will not be the only source of light and heat, but that they 

 will also be produced by the loss of motion of the solid and 

 liquid parts moving through the gases. 



This theory attempts to explain how nebulas originally came 

 into existence and assumed the shapes they actually have. And 

 in this it differs from that of Sir William Herschel, who only 

 speculated on what would be the subsequent motions and beha- 

 viour of nebulous matter after it had once been formed. 



I will now proceed to examine what account this theory ren- 

 ders of the phenomena observed in nebulas. 



And first, I think, if we admit that a great part of the matter 

 forming a nebula is invisible, we remove the immense difficulty 

 which we otherwise experience in attempting to conceive how 

 nebulse have assumed or can retain their extraordinary shapes and 

 yet be subject to the law of gravitation. 



The researches of Huggins prove that many of the nebulas 

 are gaseous self-luminous masses, whilst the spectra of others 

 prove the existence of solid or liquid matter. These last, being 

 generally those which are most easily resolved, would be those 

 which have been longest in existence, and which, therefore, should 

 contain most liquid and solid matter. 



The curious shapes of the nebulas are well explained on this 

 theory. 



Thus the spiral nebulas would be produced by two masses of 

 gas rushing together and forming an eddy in which they become 

 mingled with each other. If any detached portion of one gas 

 got mixed up with the other gas, it would be drawn out by the 

 eddy into an elongated spiral form, the surface of which would 

 become visible by the light 

 produced by the chemical ac- 

 tion. This %§T 

 would be the kind of ne- 

 bula which would be pro- 

 duced when the two masses 

 of gas did not differ greatly 

 in size, and rushed together 

 in a line not very oblique to 

 the line joining their centres 

 of gravity. 



Some spiral nebulas consist only of a single whirl. A nebula 



