The Nebular HyjMhesis : Its Present Condition, 179 



earth have been found to contahi a large number of terrestrial elements 

 — iron, in a native, uncombined form, and in great quantity; cobalt 

 nickel, sulphur, silica, in the formof augite ; molybdenum, tin, copper, 

 and a number of other metals in smaller quantity. All these are 

 substances of an entirely different character from those elements 

 identified as composing the nebula?. The former are among the most 

 refractory of chemical elements ; the latter quite defy our utmost 

 attempts to liquefy them. In both they appear to exist in an uncom- 

 bined form, though their union would go far to supply the materials 

 of a world such as ours. Some important elements, as oxygen,* are 

 wanting, it is true ; but it must be remembered that we know 

 already of two important constituents which we are unable to identify 

 with terrestrial elements, namely, the substance which at least as 

 frequently as carbon is found the sole component of comets, and the 

 gas equally known which exists along with nitrogen and hydrogen in 

 the nebulae. 



Although in the system of Laplace the cometary element held only 

 a very subordinate position, it must not be imagined that it was ne- 

 glected altogether. On the contary, Laplace, with that far reaching 

 power of generalization which is characteristic of great minds, perceived 

 that the encounter of comet with nebula was inevitable, and the 

 mechanical problem of the diminution of velocity, in consequence of the 

 former moving impeded through the gases of the latter, received from 

 him much attention. He perceived that a comet once within the power 

 of attraction of a nebula had no chance of escape, but must revolve 

 around its center of gravity in an ever diminishing spiral ; but, unac- 

 quainted with the peculiar chemical constitution of these bodies, it 

 was impossible for him to carry his speculations further. In the light 

 of modern discovery we may perceive, however, that the assimilation 

 of comets into the nebulous matter, foreseen by Laplace, will be mucli 

 more rapidly expedited, and that, notwithstanding the augmentation 

 of temperature produced by friction, a still more considerable evolu- 

 tion of heat will result from the chemical union of the two. We can 

 not pretend to explain the exact chemical efiects produced by the 

 impact, owing to our ignorance of some of the substances themselves ; 

 but, in any case, an enormous development of heat is certain to result ; 

 and whether the compound thus formed assumes the liquid or retains the 

 solid form, it will certainly bo of greater specific gravity than the rest of 

 the nebula, of which it may now be said to form a part, and the revolu- 

 tion around the center of gravity of the mass will be conducted precisely 



* Except as combined with silicon. 



