672 T. C. CHAM BERLIN 



cohesive attractions functioned effectively in the early stages; but 

 that fragmentation, abrasion, and exfoliation came into increasing 

 effectiveness as larger sizes were attained. Theoretically, then, 

 growth from the minute state at which the planetesimals started, 

 took place presumably up to limited sizes with relative facility, 

 beyond which the presumption of much larger growth was adverse, 

 except under unusual conditions. Theory, however, does not 

 define at all closely where the balance between the opposing 

 agencies was to be found, and so we turn to naturalistic evidence 

 which is more decisive. 



I. The zodiacal planetesimals. — It is an accepted view that the 

 zodiacal light is due to the reflection of solar light from minute 

 solid or liquid particles distributed in a lenslike form about the sun. 

 The central plane of the lens is essentially coincident with the 

 common plane of the planetary system. The outer border of 

 the lens reaches to some undetermined distance beyond the earth. 

 Under favorable conditions, it is possible to trace the counter- 

 glow (Gegenschein), on the side of the earth opposite the sun, into 

 continuity with the zodiacal light on the sunward side. It is not 

 improbable that the edge of the lens is extremely attenuated 

 and extends indefinitely outward in the plane of the planets. 

 The form and extent of the lens in the planetary plane make it 

 scarcely less than certain that the particles are sustained by orbital 

 dynamics, and that the orbits are of the planetary tjrpe and that 

 hence the particles are planetesimals. This warrants us in turning 

 to them for light on the sizes and masses of planetesimals. Their 

 testimony is obvious for they are certainly quite small. 



Though they envelop the earth and must in many cases be 

 quite near, the individual planetesimals are too small for detection. 

 Although they are certainly very numerous, their joint mass is 

 not known to affect the motions of any body. They are inter- 

 preted either as remnants of the original planetesimal system, or 

 as more recent products due to the projection of solar matter so 

 close to the planets that it is drawn forward by them into elliptical 

 orbits about the sun. If the first view is correct, or in so far as 

 it is correct, the planetesimals are exceptionally old and should 



