116 Trans. Acad. Sci. of St. Louis. 



1900^ C. For air the value of 6Ms 2.88 X 10« and the tem- 

 perature would be about 27000^ C, if the nebula were of air. 



These values show that at this stage of its history, if the 

 solar nebula had been a gas, the mass would practically all 

 follow the law of a perfect gas, since at Neptune's place the 

 temperatures are far" above the critical temperatures for 

 ordinary gases. 



The results also show that the effect of the outer layers, 

 which are too cool to follow the law of a perfect gas, is insig- 

 nificant so far as temperature and pressures within the great 

 body of the nebula are concerned. 



The pressures due to them are very small, and the work 

 which they are capable of doing on the entire mass during 

 contraction is correspondingly small. 



It certainly seems very probable that these equations may 

 now hold for the sun. But it is clear that they could not 

 have applied to the solar nebula while the various planets were 

 separating from the parent mass. The idea that the nebula 

 was then a gaseous nebula is utterly out of the question. The 

 direction towards which we must turn for a solution of this 

 problem is that indicated in the masterly discussion of G. H. 

 Darwin in the Philosophical Transactions of the Royal So- 

 ciety A. 1889.* The hypothesis that the material composing 

 the planets consisted of a swarm of meteoric masses is not 

 only a reasonable one, but it is the only reasonable one that 

 has been suo;o;ested in view of the numerical results which 

 appear to follow from these equations for a gaseous nebula. 



The collisions within such a swarm of meteorites would 

 result in the breaking up of those having smaller density and 

 greater brittleness more rapidly than those of a metallic charac- 

 ter. The sorting of these masses according to size, and the ac- 

 cumulation of the larger masses towards a gravitating center, 

 discussed by Darwin, explains the increase in density towards 

 the center of the earth. The recent remark of Rutherford 

 concerning the distribution of radio-active matter in the earth's 

 crust, calls in question all of the results which have been 



* Ou the Mechanical Conditions of a Swarm of Meteorites, and on Theo- 

 ries of Coemogony. 



