DIASTROPHISM AND THE FORMATIVE PROCESSES 673 



have reached the fullest growth to which they are ordinarily subject. 

 If they are of more recent origin, they merely bear testimony to 

 the common size to which planetesimals of the younger order 

 attain. But as they are so obviously minute their testimony in 

 either case is weighty. 



2. The satellitesimals of Saturn's rings. — Satellitesimals are 

 merely special forms of planetesimals. It is convenient to distin- 

 guish between them in certain cases, while in other cases the generic 

 term planestesimal is most satisfactory. In the Saturnian case, 

 they are notable for their very close association with one another 

 and for their definite borders. These hint at a special origin, per- 

 haps the disintegration of a satellite by the differential attraction 

 of Saturn, since they lie within its Roche limit. At any rate, the 

 closeness of the individual satellitesimals to one another gives them 

 rather pointed bearing on the question of growth to large sizes, for 

 their nearness to one another should favor this, if mutual attraction 

 has any appreciable effect. According to Bell's studies of their 

 albedo, they are chiefly very minute particles.' Only rarely is 

 there evidence of masses reaching as much as a meter in diameter. 

 Trituration as a consequence of their mutual collisions is probably 

 the dominant size-controlling agency in this case. 



3. Precipitate aggregates formed from condensing gases. The 

 chondrules. — ^When the gases or vapors of stony and metallic sub- 

 stances mixed with lighter gases were expelled from the sun into 

 the vacuum of interplanetary space, they must probably have 

 been greatly expanded and cooled and the stony and metallic sub- 

 stances thrown down as precipitates at successive stages according 

 as the appropriate temperatures were reached. As each gaseous 

 substance was diffused through the others, the precipitates could 

 at first have been little more than molecular in size, but by subse- 

 quent interaction, in the fashion of Brownian particles, the first 

 precipitates were brought into contact with one another and in 

 their fresh, hot, viscous states should have united into larger 

 aggregates rather freely. In so far as they solidified, they naturally 



' Louis Bell, "The Physical Interpretation of Albedo, II, Saturn's Rings," Astro- 

 phys. Jour., Vol. L (July, 1919), pp. 1-22. 



