THE NEBULAR HYPOTHESIS: ITS PRESENT CONDITION. 25 
and a number of other metals in smaller quantity. All these 
are substances of an entirely different character from those ele- 
ments identified as composing the nebulae. 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 uncombined 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 im- 
portant constituents which we are unable to identify with ter- 
restrial elements, namely, the substance which at least as fre- 
quently as carbon is found the sole component of comets, and 
the gas equally unknown 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 neglected altogether. On the contrary, Laplace, with 
that far-reaching power of generalisation which is characteristic 
of great minds, perceived that the encounter of comet with 
nebula was inevitable, and the mechanical problem of the dimi- 
nution of velocity, in consequence of the former moving im- 
peded 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 re- 
volve round its centre of gravity in an ever diminishing spiral ; 
but, unacquainted 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 the comets into the nebulous 
matter, foreseen by Laplace, will be much more rapidly expe- 
dited, and that, notwithstanding the augmentation of tempera- 
ture produced by friction, a still more considerable evolution of 
heat will result from the chemical union of the two. We can- 
not pretend to explain the exact chemical effects 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 be 
of greater specific gravity than the rest of the nebula, of which 
it may now be said to form a part, and the revolution around 
the centre of gravity of the mass will be conducted precisely as 
indicated by Laplace. Those familiar with the extraordinary 
convolutions of many of the nebulae will not fail to see how 
easily many such appearances may be explained, by imagining 
a long stream of meteoric bodies in the track of a comet pour- 
ing into the nebulous matter, and being retarded and absorbed 
Except as combined with silicon. 
