448 L, Soema7i7i on the Unity of Geological 



uted throughout the earth this would contain only 0*0042, or 

 100 times less than the least hygrometric of the feldspars. It is 

 probable that the water of the ocean thus absorbed would enter 

 into chemical combination ; at all events it would occupy a space 

 much less than the pores produced by the shrinking of the rocks. 

 If now we attempt a similar calculation for the atmosphere 

 we find that in supposing a height of eight kilometers, the total 

 volume of the air which surrounds our globe, brought to the 

 density which it has at the surface, would be about four mil- 

 lions of cubic myriameters, the volume of the earth being, 

 equal to 1083 millions, or 270 times that of the air, so that a 

 contraction of the primitive volume producing a vacuum of 

 four thousandths (^^q) would be more than suflBcient to ab- 

 sorb the whole of the atmosphere. (In calculating the volume 

 of the atmosphere we have multiplied the surface of the globe in 

 square myriameters, by 0-8, which gives a sufficiently accurate 

 result, the more so that the density of the air in the interior of the 

 €arth will be everywhere greater than at the surface.) 



It now remains to be seen whether the assumption of a shrink- 

 ing of four thousandths can be justified by analogies. In the want 

 of direct determinations of the porosity of crystalline rocks, upon 

 which subject I am not aware of any published experiments, the 

 observation upon the fusion of rocks, and the determinations of 

 their densities in the crystalline and vitreous states admit of an in- 

 direct application to the question before us. The experiments of 

 Charles Ste. Claire Deville in the Comptes Renclus for 1845, and 

 of Del esse in the Bulletin for 184'7, agree so closely in this mat- 

 ter that we give them the preference over those of Bischof, pub- 

 lished in 1842. Deville and Delesse found that the fusion of 

 rocks yields glasses whose densities are generally inferior to that of 

 the rock in the crystalline state. This diminution for granite is 

 equal to from nine to eleven hundredths, and it is evident that 

 such a glass passing to a crystalline state and retaining its volume, 

 must present vacant spaces in direct proportion to the augmenta- 

 tion of density, that is to say, equal to about one-tenth of its vol- 

 ume. If wre take the mean density of granite at 2-60, it might, 

 with such a degree of porosity, imbibe 3*9 parts in lOO'O of its 

 weight of water. This shrinking of one-tenth is no exaggeration, 

 and such a rock would still be a good building material, aUhough 

 containing twenty-five times more vacant space than our calcula- 

 tion requires. 



