236 CARNEGIE INSTITUTION OF WASHINGTON. 



pores and other interstices of the fragmental surface arising from 

 the infall of the solid planetesimals. 



The extent to which gases may be held condensed in small solid 

 bodies is shown by meteorites and igneous rocks to be large. Mete- 

 orites carry on the average several times their volume of condensed 

 gas ; so do many, probably most, igneous rocks of the earth. The 

 testimony of the meteorites is peculiarly significant here, for they 

 have traversed unknown depths of space in a practical vacuum, in 

 addition to the vicissitudes of their origin and the heating of their 

 fall. Atmospheric material is carried into the earth's body by them 

 today in quantities that are large relative to their masses. Their 

 testimony becomes the more significant if we accept the view of 

 their origin which makes them but the fragments of small atmos- 

 phereless bodies, built up precisely as the early earth was under this 

 hypothesis. This view makes them specific samples of the products 

 of the assigned process. 



The atmospheric material thus condensed within the growing earth 

 could become a part of the atmospheric envelope only by extrusion. 

 The assigned modes of extrusion will be considered presently ; mean- 

 while it may be assumed that these internal gases were given forth 

 progressively and fed the atmosphere. 



The contribution made by the external sources of atmospheric 

 material might include any constituent of the ancestral sun that 

 could remain free in the nebula and be picked up and held by the 

 earth. Some portion of the constituents of the present atmosphere 

 may therefore be assigned to this source. In what ratio these con- 

 stituents were contributed to the nebula probably depended on their 

 proportions in the ancestral sun, or rather their proportions in that 

 part of the ancestral sun that was dispersed to form the parent nebula. 

 Concerning this little can safely be said. Hydrogen is apparently 

 very abundant in the other part of the sun, but it is doubtful whether 

 the earth can even now hold hydrogen in a free state permanently 

 in any large amount. Of the proportions of the common atmospheric 

 constituents in the sun in a free state little is known. 



The gases chiefly occluded in meteorites and the crystalline rocks 

 are hydrogen, carbon dioxide, and carbon monoxide in leading 

 amounts, and marsh-gas and nitrogen in small quantities. It is as- 

 sumed that the gases of the aggregated planetesimals, and hence those 

 of the interior of the early earth, were of the same order of abun- 

 dance. There is experimental ground for believing that, at the right 

 temperatures and pressures, hydrogen would take oxygen from ferric 



