Iviii REPORT — 1866. 



mineral character, and which, by involving secondary oxygenized compounds, 

 mustalsochangetheirchemieal constitution. By experiments he has succeeded 

 in forming from terrestrial rocks substances very much resembling meteorites. 

 Thus close relationship, though by no means identity, is established between 

 this earth and those wanderers from remote regions, some evidence, though 

 at present incomplete, of a common origin. 



Surprise has often been expressed that, while the mean specific gravity of 

 this globe is from five to six times that of water, the mean specific gravity of 

 its crust is barely half as great. It has long seemed to me that there is no 

 ground for wonder here. The exterior of our planet is to a considerable 

 depth oxidated ; the interior is in all probability fi-ee from oxygen, and what- 

 ever bodies exist there are in a reduced or deoxidated state, if so, their specific 

 gravity must necessarily be higher than that of theii- oxides or chlorides, &c.: 

 we find, moreover, that some of the deep seated minerals have a higher 

 specific gravity than the average of those on the surface ; olivine, for instance, 

 has a specific gravity of 3'.3. There is therefore no a priori improbability that 

 the mean specific gravity of the earth should notably exceed that of its 

 surface ; and if we go further and suppose the interior of the earth to be 

 formed of the same ingredients as the exterior, minus oxygen, chlorine, 

 bromine, &c., a specific gravity of 5 to 6 would not be an imlikely one. 

 Many of the elementary bodies entering largely into the formation of the 

 earth's crust areas lighter lighter than water, — for instance, potassium, so- 

 dium, &c.; others, such as sulphur, silicon, aluminium, have from two to three 

 times its specific gravity ; others, again, as iron, co^jper, zinc, tin, seven to 

 nine times ; while others, lead, gold, platinum, &c., arc much more dense, — 

 but, speaking generally, the more dense are the least numerous. There seems 

 no improbability in a mixtiu-e of such substances producing a mean specific 

 gravity of from 5 to 6, although it by no means follows, indeed the proba- 

 bility is rather the other way, that the proportions of the substances in the 

 interior of the earth are the same as on the exterior. It might be Avorth the 

 laboiir to ascertain the mean specific gravity of all the known minerals on the 

 earth's surface, averaging them in the ratios in which, as far as our know- 

 ledge goes, they quantitatively exist, and assuming them to exist -ndtho^it the 

 oxygen, chlorine, &c., with wliich they are, A^'ith some rare excejjtions, 

 invariably combined on the surface of the earth : great assistance to the know- 

 ledge of the probable constitution of the eartli might be derived from such an 

 investigation. 



While chemistry, analytic and synthetic, thiis aids us in ascertaining the 

 relationship of our planet to meteorites, its relation in composition to other 

 planets, to the sun, and to more distant suns and systems is aided by another 

 science, vi'i. optics. 



That light passing from one transparent medium to another should carry 

 with it evidence of the soiu'ce from which it emanates, would, untU lately, havo 

 seemed an extravagant supposition ; but probably (could we read it) every- 

 thing contains in itself a large portion of its own history. 



I need not detail to you the discoveries of Kirchhoff,Eunsen,MiUcr,Huggins, 

 and others, they have been dilated on by my predecessor. Assuming tliat 

 spectrum analysis is a rehable indication of the presence of given substances 

 by the position of transverse bright lines exhibited M-hcn they are burnt and 

 of transverse dark lines when light is transmitted through their vapours, 

 though Plucker has shown that with some substances these lines vary Avith 

 temperature, the point of importance in the view I am presenting to you is, 

 that while what may be called comparatively neighbouring cosmical bodies 



