GEOLOGY. 375 



ing or growth had changed the holding power of the parent body. 

 Planetesimals from both sources were liable to cross the paths of nuclei 

 at any revolution and so be subject to capture. Such capture doubt- 

 less began to take place as soon as they returned from their first ex- 

 cursions; later it was liable to take place only on such periodic returns. 



The size and rate of infall of planetesimals are important in this 

 discussion because they have bearings on the Uquidity or solidity of the 

 earth, and because it has been urged that impacts of large planetesimals 

 or planetoids may have caused the pitted nature of the moon's surface. 

 The size and rate of infall of planetesimals have therefore been care- 

 fully reconsidered, giAdng special attention to the hght thro\\Ti on the 

 subject by their dynamic conditions. The subject is much clarified 

 if it is noted that at the outset essentially all planetesunals were either 

 separate molecules or minute precipitate-aggregates projected dkectly 

 from the solar outbursts into orbits of the planetai-y typ^, or else were 

 separate molecules tliat escaped from nuclei of limited gravity. Theu* 

 sizes at later stages were determined by such growths as could arise 

 from chance accession or from the union of these minute initial bodies 

 with one another while in their orbital flights. There was no con- 

 densation of the gaseous type, for each planetesimal was under the con- 

 trol of a moving force so high as to render their mutual attractions 

 relatively insignificant. Only under rare conditions — chiefly when a 

 delicate balance of the higher order of forces occurred — could mutual 

 attraction have any appreciable effect. Electric attractions and repul- 

 sions probably helped aggregation rather notably up to certain sizes, 

 but it was probably ineffectual beyond. Colhsions and triturations 

 worked against growth probably more than for it, and exfoliation due 

 to changes of temperature tended to reduce their masses after they 

 became large enough to suffer from differential heating. The satel- 

 litesimals of Saturn's rings, the largest of which Bell places at 3 meters 

 or thereabouts, are probably the best naturahstic guide to a defi- 

 nite judgment now available.-^ 



It is an error to suppose that the planetesimal theory postulates a 

 gradation from planetesimals to planetoids. The latter are formed in 

 a distinctly different way, by the concentration of nuclei. Planetoids 

 probably represent the smallest class of bodies that could be formed in 

 this concentrative way in the inner portion of the sun's sphere of con- 

 trol where its gravitative stresses are adverse. The inquiry shows 

 that none of the atmosphereless bodies could be formed du'ectly 

 by gaseous condensation; they were probably formed by the indirect 

 precipitate-aggregate method. This view is supported by the fact 

 that no planetoids are observed as near the sun as the earth. They 

 appear first at about the distance of Mars, where the dynamic environ- 



'Louis Bell, Physical interpretation of Albedo II: Saturn's Rings. Astrophys. Jour., vol. 50, 

 1-22, July 1919. 



