62 



been computed that if the Andromeda nebula had a density 20.000.000 

 thai of the sun it would have mass enough to attract tin- earth as Btrongly 

 as tin -uii does. It attracts the earth not at all. Nor does it attract any 

 other body as far as we know, many of them being much closer to it tie 

 we are. 



We do not know the chemical composition of the nebulae, except that it 

 see is to be different from every thing else in the sky. Not one ha- 

 been seen to change its shape, size ot brightness. We have always assumed 

 that stars result from the contraction of nebulae and this is based on the idea 

 that the nebulae radiate heat. It is not at all certain that these rare gases 

 shine because of their heat. A mass of gas of such extreme rarity would have 

 a comparatively small amount of heat and it would seem that this ought to 

 be radiated into space very rapidly, and could not be miantained without 

 rapid contraction. It is quite possible that nebular matter instead of being 

 the raw material of stars and planets is matter in some final form after 

 having gone through its life history. We have no observational data either 

 way and will probably not have any for many centuries to come. There 

 does not seem to be any very good reason if or believing that matter is not 

 being created now as much as it ever was nor for thinking that it must always 

 endure in some of the forms we now know. 



We think of space as infinite in extent. Whether or not matter, in the 

 forms we know, is to be found in all parts of space, we do not know. That 

 is to say we are not yet sure whether the universe is finite or infinite. There 

 are some reasons for thinking that the system of the stars is as infinite as 

 space itself, but it may also be possible that what we call matter is some mani- 

 festation peculiar to this part of space. The mere appearance or disappear- 

 ance of matter in space would in itself be no more remarkable than the 

 precipitation and evaporation of water would be if we knew nothing of the 

 atmosphere, and perhaps not as remarkable as the production of water 

 from two invisible and unknown gases would seem to people who know nothing 

 of chemistry. 



The most probable source of information it seems to me. will be the 

 researches of the physicists and chemists on the real nature of matter. When 

 they shall have told us what matter really is. what all of its possible forms 

 may be and what aU the sources of energy are. then we may be able to state 

 with certainty what the life history of a star is. what relation the nebulae 

 have to other bodies, and what in reality has been the past history of our 

 planet and other planets. 



