10 Dr. J. Croll on the Origin of Nebula. 



Binary, Triple, and Multiple systems of stars will of course 

 be accounted tor in a similar maimer. 



It is conceivable that it may sometimes happen that by the 

 time the materials are broken up and dissipated into space, 

 there may not be sufficient heat left to convert the fragments 

 into vapour. In this case we should have what Professor 

 Tait has suggested, a nebula consisting of " clouds of stones." 

 But such nebulae must be of rare occurrence. 



Objections considered. — On a former occasion I considered 

 one or two anticipated objections to the theory that stellar 

 light and heat were derived from motion in space. But as 

 these objections have since been repeatedly urged by physicists 

 both in this country and in America, I shall again briefly refer 

 to them. 



Objection 1st. " The existence of such non-luminous bodies 

 as the theory assumes is purely conjectural, as no such bodies 

 have ever been observed." In reply, it is just as legitimate 

 an inference that there are bodies in stellar space not luminous 

 as that there are luminous bodies in space not visible. We 

 have just as good evidence for believing in the existence of 

 the one as we have in the existence of the other. Bodies in 

 stellar space can only be known through the eye to exist. If 

 they are not luminous, they of course cannot be seen. But 

 we are not warranted on that account to suppose that they do 

 not exist, any more than we have to suppose that stars do not 

 exist which are beyond the reach of our vision. We have, 

 however, positive evidence that there are bodies in space non- 

 luminous, as the meteorites and planets for example. The 

 stars are beyond doubt suns like our own ; and we cannot 

 avoid the inference that, like our sun, they are surrounded by 

 planets. If so, then we have to admit tlfat there are far more 

 bodies in stellar space non-luminous than luminous. But 

 this is not all: the stars no more than our sun can have been 

 dissipating their light and heat during all past ages ; their 

 light and heat must have had a beginning ; and before that 

 they could not be luminous. Neither can they continue to 

 give out light and heat eternally ; consequently when their 

 store of energy is exhausted they will be non-luminous again. 

 Light and heat are not the permanent possession of a body. 

 A body may retain its energy in the form of motion undimi- 

 nished and untransformed through all eternity, but not so in 

 the form of heat and light. These are forms of energy which 

 are being constantly dissipated into space and lost in so far as 

 the body is concerned. 



The conclusion to which we are therefore led is that there 

 are in all probability bodies in stellar space which have not 



