DIASTROPHISM AND THE FORMATIVE PROCESSES 145 



ejective activity stimulated to special intensity by the dififerential 

 attraction of some passing body. Two essential factors are involved : 

 (i) the ejection to the requisite distance of the requisite matter — 

 only a fraction of i per cent of the sun's mass; and (2) the addition 

 of sufficient transverse momentum to cause the ejected matter to 

 revolve about the sun. The latter is the more critical factor, 

 for the eruptivity of the sun is known to have such a high degree 

 of efficiency, even at the present time, that only a relatively sHght 

 increase is required to project the small fraction of the sun's 

 substance to the distance of the planets. Such projections would, 

 however, fall back to the sun, unless they were given a transverse 

 motion by some agency other than radial projection. A passing 

 star or other body has been postulated to meet this requirement. 

 The tidal stresses developed in the sun by such passing body would 

 stimulate eruptivity and give direction to the projections while the 

 pull of the passing body would draw the ejected matter into orbital 

 courses. 



In about a half-hundred cases worked out mathematically by 

 Moulton to test the vaHdity of the postulated effects, a star of 

 medium size passing at from one to five astronomical units' distance 

 was taken as the parent of the orbital motion; its diverting com- 

 petency was found to be unexpectedly effective.^ 



Later it was suggested that a much smaller body — passing how- 

 ever much nearer to the sun — ^might serve as well to give both the 

 tidal stimulus and the transverse motion required, but this has not 

 yet been worked out mathematically.^ Still more recent studies 

 ha,ve led to the behef that there is a wide range of possibiHties 

 respecting the co-operating body, as will be specified later. 



RECENT DISCLOSURES BEARING ON THE SOLAR PARENTAGE 

 OF THE PLANETS 



Respecting the projectile power of the sun, important light 

 has been shed by very recent discoveries. Remarkable eruptions 

 of the sun took place on May 29 and on July 15, 1919. A fine 

 series of spectroheHographic photographs were taken at the Yerkes 



' Carnegie Year Book, No, 5 (1906), pp. 166 and 168. 

 ' The Origin of the Earth (1916), p. 118. 



