The Nebular HijjMhesis : Its Present Condition. 181 



in some cases at least, more closely approximates to the true condition 

 of things. It is well known that the great nebula in Andromeda — 

 one of those which defy alike the powers of the telescope and spec- 

 troscojje — presents a very anomalous spectrum. Though apparently 

 continuous, some portions of the red and orange are altogether wanting, 

 and the more refrangible portion consists of a mottled band, with 

 uneven gradation of intensity. This is very much what would be 

 expected if the spectrum were really one of numerous bright lines, nor 

 is it a singular instance. Several other nebulte have given rise to 

 strong suspicions of a similar constitution ; and no theory, so fur as I 

 am aware, has hitherto been advanced in explanation of these appear- 

 ances. I need not point out that if this supposition is correct, a state 

 of things arises in which the nebular hypothesis in its later form will 

 again apply, with the addition only that the gases of the nebula? are in 

 a much more complicated condition than was formerly supposed to be 

 the case. 



It is to be remarked that the number of comets Avhich would be 

 attracted to a nebula would increase in proportion as its mass increased 

 with each succeeding capture, so that its development would proceed 

 at an accelerated fate until a certain point was reached, when com- 

 paratively little of the gaseous elements remain. The comets would 

 then describe their orbits around the newly formed sun, and Avould 

 leave only their smaller or outlying meteorites to swell its mass. But 

 even these would greatly tend to compensate for the dissijiation of 

 heat by radiation, and would much retard the cooling process — a state 

 of things that will continue to exist even in a perfectly formed system, 

 such as our own. I am thus led to regard the meteoric theory of the 

 supply of solar heat as a part of the modified nebular hypothesis which 

 I have suggested. If our sun is a nebulous star, surrounded by a far- 

 extending atmosphere, in which are revolving a large number of 

 meteoric bodies, visible possibly to us, as the Zodiacal Ijight, the ma- 

 terials for keeping up a constant or nearly constant te^iperature for a 

 considerable length of time are at hand, and every succeeding comet 

 will add soiAe to the number of those meteors which, unlike it, are 

 unable to make their final escape, and pursue their orbits with scarcely 

 diminished velocity. The smaller meteors in a stream, as presenting 

 relatively a larger surface, will be mor% retarded than the larger, and 

 will more rapidly fall upon the solar surftice — so rapidly, indeed, that 

 it does not seem unlikely that the universally held belief in the 

 increased solar heat during years noted for large comets may have a 

 sound foundation. Such a possibility is sufficient to invest the 

 movements of comets with great practical interest, and the various 



