MEMOIR ON METEORITES. 303 



some grounds for connecting meteorites with the planets. 

 Among the planets there is also a difference, and a very 

 marked one, in their respective densities ; Saturn having a 

 density of 0.77 to 0.75, water being 1.0; Jupiter, 2.00-2.25; 

 Mars, 3.5-4.1; Yenus, 4.8-5.4; Mercury between 7 and 36; 

 Uranus, 0.8-2.9; that of the Earth being 5.67.* If then from 

 specific gravity we are to connect meteorites to the planets, as 

 their mean density is usually considered about S.0,f they must 

 come within the planetary range of Mars, Earth, and Yenus. 

 In the cases of the first and last we can trace no connection, 

 from our ignorance of their nature and of the causes that could 

 have detached them. 



This reduces us then to our own planet, consisting of two 

 parts, the planet proper, with a density of 5.76, and the moon, 

 with a density of about 3.62.J On viewing this, we are at 

 once struck with the relation that these bear to the density 

 of meteorites, a relation that even the planets do not bear to 

 each other. 



As before remarked, I lay no great weight on this view of 

 the density, but call attention to it as agreeing with conclusions 

 arrived at on other grounds. 



The chemical composition is also another strong ground in 

 favor of their. lunar origin. This has been so ably insisted on 

 by Berzelius and others that it would be superfluous to attempt 

 to argue the matter any further here; but I will simply make 

 a comment on the disregard that astronomers usually have 

 for this argument. In the memoir on the periodic meteors by 

 Sears C. Walker, already quoted from, it is stated, "The chem- 

 ical objection is not very weighty, for we may as well suppose 

 a uniformity of constituents in cosmical as in lunar substances." 

 From this conclusion it is reasonable to dissent, for as yet we 

 are acquainted with the materials of but two bodies, those of 



*Por these estimates of the density of the planets the author is indebted 

 to Prof. Peirce. 



t Although the average specific gravity of the metallic and stony meteor- 

 ites is greater, yet the latter exceeding the former in quantity, the number 

 3.0 is doubtless as nearly correct as can be ascertained. 



± Although the densities of the earth and moon differ, these two bodies 

 may consist of similar materials, for the numbers given represent the density 

 of bodies as wholes. The solid crust of the earth for a mile in depth can 

 not average a density of 3.0. 



