G42 



SCIENCE 



[N. S. Vol. XXX. No. 775 



THE DEVELOPMENT OF THE PLANETESIMAL 

 HYPOTHESIS 



Wpien, in 1906, tte planetesimal hypothesis 

 had reached a stage of development sufficient 

 to warrant its introduction as a working hy- 

 pothesis into text-books of geology and astron- 

 omy, it seemed to its authors worth while to 

 draw up and place on their private files a 

 memorandum of the several stages of cos- 

 mogonic study that had led up to the hypoth- 

 esis in the form it had then taken. It was 

 not assumed that the hypothesis had reached 

 a final form, much less that it was in any 

 sense then proven or that could approach proof 

 until after a long period of trial and the 

 closest scrutiny. On the contrary, they were 

 then engaged in further efforts to test its 

 working qualities and to add to its details or 

 to modify them. It, however, seemed worth 

 while at that stage to make note of preceding 

 steps of progress while fresh in mind for 

 future reference if occasion should require. 

 Such occasion seems now to have arisen. 



In the introduction to the memorandum, 

 by way of qualifying the statements of the in- 

 dividual parts taken, it was noted that the 

 mutual studies of the authors had grovsm up 

 so gradually and informally, their conferences 

 had been so frequent and so free, and their 

 relations so intimate that it was difficult to 

 set down with accuracy the precise parts con- 

 tributed by each, or the aid rendered each to 

 the other in working these out. The memo- 

 randum was intended to indicate merely the 

 main individual lines of work and the lead- 

 ing stages of progress. A quite accurate and 

 detailed history could be worked out, if it 

 were worth while, from the note-books of 

 advanced students of the University of Chi- 

 cago from 1892 onward, as they were familiar 

 with the status these studies had reached at 

 the times their lecture notes were taken. 

 Several of these students made computations 

 or rendered other aid sufficient to call for 

 notice in the papers published, among whom 

 were A. W. Whitney, H. L. Clarke, J. P. 

 Goode, H. r. Bain, S. Weidman, C. F. Tol- 

 man, Jr., N. M. Fenneman, C. E. Siebenthal, 

 E. T. Chamberlin and W. H. Emmons. 

 In the synopsis below, the memorandum 



of March 12, 1906, is followed in the main, 

 but the abbreviated phrases and references 

 have been rounded out or recast to make them 

 more specific and the whole brought down 

 to date. 



I. Destruction (in the Main) 

 Line of Approach and First Step. — To find 

 out what effects on geological climates might 

 be assignable to changes in the constitution 

 of the atmosphere, Chamberlin, in the middle- 

 nineties of the recent century, attempted to 

 test, by means of the molecular velocities in- 

 volved after the method of Johnstone Stoney, 

 the probable limits to the extent of the atmos- 

 pheres in early geological stages, particu- 

 larly those conditioned by the molten and* 

 gaseous states of the early earth as then com- 

 monly postulated. 



These tests were found to throw doubt on 

 the common belief in the enormous extent of 

 hot vaporous atmospheres supposed to prevail 

 during the gaseous and molten states of the 

 earth. The test was then carried back to the 

 earth-moon ring postulated by the Laplacian 

 hypothesis where its application seemed fatal 

 to the hypothesis. Moulton aided in his test 

 by preparing tables of parabolic velocities for 

 the earth at various heights above its surface- 

 and at different rates of rotation. Dr. A. W. 

 Whitney made computations relative to molec- 

 ular velocities under varying temperatures 

 and pressures. The results were set forth in a 

 paper read by Chamberlin at the Toronto 

 meeting of the British Association for the 

 Advancement of Science, August 20, 1897, 

 and more fully in the Journal of Geology, Oc- 

 tober-November, 1897, pp. 653-683. 



Second Step. — The conclusion that the nebu- 

 lous matter of the supposed earth-moon ring 

 could not remain in a true gaseous state, i. e.^ 

 with the molecules in active collisional rela- 

 tions to one another, under the conditions 

 postulated for the earth-moon ring under the 

 Laplacian hypothesis, led Chamberlin to con- 

 sider the alternative conception of molecules 

 or particles revolving in independent orbits in 

 planetoidal fashion. Condensation from this 

 state had previously been held, generally if 

 not universally, to give rise to retrograde 



