2 4 2 THE POPULAR SCIENCE MONTHLY. 



these are to be regarded as exceptional phenomena. It is certainly true 

 that they are perhaps the most remarkable instances in which changes 

 in nebulae have actually been witnessed ; but the probability is that the 

 only reason why they have been witnessed is because they were very 

 exceptional. Those who have observed the nebulae for many years are 

 well assured of the general permanence of their appearance. The 

 nebulae we have referred to are chosen out of thousands. The ordinary 

 nebulae appear just as constant as the ordinary bright stars. Every 

 one expects to see Vega in the constellation Lyra ; and with equal con- 

 fidence every astronomer counts on seeing the celebrated annular nebula 

 when he directs his telescope to the same constellation. This perma- 

 nence is very probably merely due to the stupendous distances at which 

 these objects are placed. Only gigantic changes could be detected, and 

 for these, gigantic periods of time would be required. We are bound 

 to believe that heated bodies radiate their heat ; and, if so, they must 

 contract. This general law, which pervades all Nature, so far as we 

 know it, seems to be the real basis indeed, the only basis on which 

 the nebular theory of Herschel can be maintained. Up to the present, 

 it must be admitted that this theory has received no direct telescopic 

 confirmation. 



The nebular theory by which Laplace sought to account for the 

 origin of the solar system seems, from the nature of the case, to be 

 almost incapable of receiving any direct testimony. "We shall here 

 enunciate the theory in the language of Professor Newcomb : 



The remarkable uniformity among the directions of the revolutions of the 

 planets being something which could not have been the result of chance, Laplace 

 sought to investigate its probable cause. This cause, he thought, could be noth- 

 ing else than the atmosphere of the sun, which once extended so far out as to 

 fill all the space now occupied by the planets. He conceives the immense vapor- 

 ous mass forming the sun and his atmosphere to have had a slow rotation on its 

 axis. The mass, being intensely hot, would slowly cool off, and as it did so 

 would contract toward the center. As it contracted, its velocity would, in 

 obedience to one of the fundamental laws of mechanics, constantly increase, so 

 that a time would arrive w T hen, at the outer boundary of the mass, the centrifu- 

 gal force due to the rotation would counterbalance the attractive force of the 

 central mass. Then those outer portions would be left behind as a revolving 

 ring, while the next inner portions would continue to contract, until at their 

 boundary the centrifugal and attractive forces would be again balanced, when a 

 second ring would be left behind, and so on. Thus, instead of a continuous 

 atmosphere, the sun would be surrounded by a series of concentric revolving 

 rings of vapor. 



Now, how would these rings of vapor behave? As they cooled off, their 

 denser materials would condense first, and thus the ring would be composed of a 

 mixed mass, partly solid and partly vaporous, the quantity of solid matter con- 

 stantly increasing and that of vapor diminishing. If the ring were perfectly 

 uniform this condensing process would take place equally all around it, and the 

 ring would thus be broken up into a group of small planets like that which we 

 see between Mars and Jupiter. But we should expect that, in general, some 



