250 THE NEBULAE HYPOTHESIS. 



rashly espoused by sundry astronomers, that the nebulae 

 are extremely remote galaxies ; let us consider whethej 

 the various ajDpearances they present are not reconcile 

 able with the Xebular Hypothesis. 



Given a rare and widely-diffused mass of nebulous mat- 

 ter, having a diameter, say as great as the distance from 

 the Sun to Sirius,* what are the successive changes that 

 will take place in it ? Mutual gravitation will approxi 

 mate its atoms ; but their aj^proximation will be opposed 

 by atomic repulsion, the overcoming of which implies the 

 evolution of heat. As fast as this heat partially escapes by 

 radiation, further approximation will take j^lace, attended 

 by further evolution of heat, and so on continuously : the 

 processes not occurring separately as here described, but 

 simultaneously, uninterruptedly, and with increasing ac- 

 tivity. Eventually, this slow movement of the atoms to- 

 wards their common centre of gravity, will bring about 

 phenomena of another order. 



Arguing from the known laws of atomic combination, 

 it will happen that when the nebulous mass has reached a 

 particular stage of condensation — when its internally-situa- 

 ted atoms have approached to within certain distances, 

 have generated a certain amount of heat, and are subject 

 to a certain mutual pressure (the heat and pressure both 

 increasing as the aggregation progresses) ; some of them 

 will suddenly enter into chemical union. Whether the 

 binary atoms so produced be of kinds such as we know, 

 which is possible ; or whether they be of kinds simpler 

 th94:i any we know, which is more probable ; matters not 

 to the argument. It suffices that molecular combination 

 of some species will finally take place. When it does take 



* Any objection made to the extreme tenuity this involves, is met by 

 the calculation of Xewton, Avho proved that were a spherical inch of air 

 removed four thousand miles from the Earth, it would expand into a 

 sphere more than filling the orbit of Saturn. 



