76 THE POPULAR SCIENCE MONTHLY. 



it has "been supposed, by tidal action, as they approach the sun ; 

 because such an action must make a considerable difference in 

 the movements of the particles of the swarm nearer the sun, as 

 compared to those farther away from it ; we know, in any case, 

 by their increased light, that the temperature of comets does in- 

 crease considerably as the sun is approached. It has been shown 

 that many of the phenomena presented by comets, which are 

 acknowledged to be clouds of meteoritic particles in the solar 

 system, are identical with those presented by nebula? and stars 

 in space ; hence the hypothesis now under consideration, which 

 affirms the nebulae to be also clouds of meteoritic dust, is greatly 

 strengthened. Indeed, if the facts had not been found to be as I 

 have stated them, the hypothesis would have been worth nothing. 



I should here add that the recent work has shown how right 

 Schiaparelli was, when, in 1866, he stated that comets were nebu- 

 lous masses drawn into the solar system. 



The top of what we agreed to call the temperature curve may 

 now be considered. We have dealt with the ascending arm of it, 

 and referred to the groups I, II, and III. In these groups there 

 was evidence to show that, under normal conditions, we were 

 dealing with orders of celestial bodies in which the temperature 

 was gradually increasing, in consequence of the continual nearing 

 of the constituent meteorites in the swarm due to collisions and 

 gravitation. 



It may be convenient that I should very briefly give, even at 

 the risk of being charged with repetition, a normal case carrying 

 us up to the top of the curve. For that purpose we may con- 

 tent ourselves by considering those globular and elliptic nebulas 

 first recorded by Sir "William Herschel in the last century. In 

 these there is evidence of different stages of condensation ; in one 

 series first of all something which is hardly visible is noted, and 

 the end of that series consists of a dim, diffused, globular mass. 

 In another we pass from the minimum gradually into another 

 form of condensation, in which the luminosity increases toward 

 the center. In still another series the condensation toward the 

 center goes as it were by jumps, so that finally what appears to be 

 a nebulous star with a surrounding of very nearly equal density 

 is seen. Passing from these forms we come to elliptic nebula?, 

 which doubtless indicate a further condensation of those forms 

 which, in the first instance, are globular. We have already be- 

 come familiar with a representative of these elliptic nebula? in 

 that of Andromeda, as it has been revealed to us by the magnifi- 

 cent photograph taken by Mr. Roberts. In connection with such 

 an elliptic figure we often get clear indications of spirals. 



A further condensation then will no doubt land us among stars 

 having a peculiar and special spectrum ; indeed, though they ap- 



