190 WARREN UPHAM, M.A., F.G.S.A., ON THE NEBOLAR AND 



" Under the typical form of the planetesimal hypothesis it is 

 assumed that the parent nebula of the solar system consisted of 

 innumerable small bodies, planetesimals [infinitesimal planetoids], 

 revolving about a central gaseous mass, somewhat as the planets do 

 to-day. The hypothesis, therefore, postulates no fundamental 

 change in the system of dynamics after the nebula was once formed, 

 but only an assemblage of the scattered material. . . . 



" An inquiry into the possible modes by which the planetesimal 

 condition might arise revealed several possible methods. Such 

 condition might arise from a nebula that was originally gaseous. 

 If, for example, it be supposed that the parent nebula was a 

 gaseous spheroid, and that it detached material from its equatorial 

 belt molecule by molecule, rather than by rings, as postulated by 

 Laplace, these molecules would probably become planetesimals instead 

 of members of a true gaseous body . . . There is reason to 

 believe that this method would really be almost the only systematic 

 •one by which a gaseous spheroid of the Laplacian type would 

 •detach material from its equatorial belt. . . . 



" . . . To develop the hypothesis as definitely and concretely 

 . as possible, I have further chosen a special case from among those 

 that might possibly arise, the case in which the nebula is supposed 

 to have arisen from the dispersion of a sun as a result of close 

 approach to another large body. The case does not involve the 

 origin of a star nor even the primary origin of the solar system, 

 but rather its rejuvenation and the origin of a new family of planets 

 . . . This particular sub-hypothesis was selected for first 

 development (1) because it postulates as simple an event as it seems 

 possible to assign as the source of so great results ; (2) because that 

 -event seems very likely to have happened; (3) because the form of 

 the nebula supposed to arise in this way is the most common form 

 known, the spiral ; and (4) because spectroscopic observations seem 

 at present to support the constitution assigned this class of nebulae 



"The continuous spectrum is interpreted to mean that their 

 chief luminous material is in a liquid or solid state. . . . 

 As the liquid condition is limited to a rather narrow range of 

 temperature, and as this range is very different for different material, 

 it is improbable that any large portion of a nebula is in this state, 

 and the whole may be conveniently treated as though it were formed 

 of solid matter, but matter in a finely divided condition. This last 

 qualification seems necessary, for the volume of these nebulae is often 

 very great, and yet they appear to intercept but little light and 

 give no signs of great attractive power. 



" The prevailing form of these nebulae is the spiral, as determined 

 by the late Professor Keeler, and this form particularly characterizes 

 the smaller nebulae recently brought to knowledge by improved 

 instruments and manipulative skill. Those newly discovered 



