President's A ddress. 
119 
irresolvable nebulae in the separate individuals of which a 
central condensation and brightness v^as observed gradually 
to increase until it assumed a star-like character, Herschel 
inferred that physical changes v^ere actually taking place 
in a thin luminous haze capable of receiving impulses ; and 
that this might be the process by which the original ele- 
ments of matter were condensed into a brilliant sun. This 
was the basis of the celebrated " Nebular hypothesis/' after- 
wards extended by La Place, who not only applied it to 
account for the creation of suns from self-luminous vapours, 
but for the formation of planets from rings of nebulous 
matter thrown off from these suns during the process of 
condensation, and of satellites from the planets as they in 
turn became solidified. The earth's crust is found to be 
almost entirely composed of four minerals — silica, alumina, 
magnesia, and lime ; but each of these minerals consists of 
the elements silicium, aluminium, magnesium, and calcium, 
combined with about 43 per cent, of oxygen. It is known 
that very high temperatures are unfavourable to the union 
of oxygen with other elements, and that a great heat can 
convert every mineral known substance into vapour ; and 
taking these facts along with another — namely, that the 
components of the rocky strata which constitute the crust 
of the earth would in a separate state occupy 2000 times 
the space they now occupy — we shall at once perceive that, 
under the action of the high primeval temperature supposed, 
this globe would put on an appearance not very unlike that 
presented by the nebulous stars to the astronomer. The 
nebular hypothesis assumes that before the arrangement of 
the earth into that complex and beautiful structure it now 
presents, it consisted of a round mass of incandescent vapour 
hung in space, uninfluenced by any force except the gravi- 
tation of its parts to each other. This force of gravitation 
would not act equally on all its parts, but would affect some 
to a greater degree than others ; and the consequence of 
this would be that the particles would arrange themselves 
in the ratio of their respective densities. There would be 
accumulated round the central nucleus the fluid matter 
which now constitutes the interior part of the earth, and 
