President's A ddress. 



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irresolvable nebulae in the separate individuals of which a 

 central condensation and brightness was observed gradually 

 to increase until it assumed a star-like character, Herschel 

 inferred that physical changes were 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 



