1907-1908.] NebulcB and Nebular Hypotheses, 7 ' 



cooler as it becomes older. But cooling is always accompanied 

 by contraction — the two proceed fari passu. In fact, " in all 

 cosmical bodies there is a perennial contest between heat and 

 gravity. Heat strives for expansion, gravity for contraction. 

 Heat drives the components of a mass outwards and apart; 

 gravity draws them inwards and closer together. And gravity 

 must in all cases win in the end, for heat is transient and 

 gravity everlasting." We are therefore led to surmise that 

 old stars must be small stars. As they cool they consolidate. 

 Contrariwise, young stars are likely to be large and tenuous, 

 because their great heat expands them, and, so to speak, puffs 

 them out. Here, then, we have another criterion for stellar 

 age or youth. Our first was one of temperature ; our second, 

 a direct consequence of the first, is one of density. Young 

 stars are diffuse, and their density is small. We should ex- 

 pect, therefore, that if there exists in space any material out 

 of which the stars are formed, that material should be very 

 diffuse ; it should, in fact, be cloudlike, or, to use the Latin 

 form of the word, it should be nebular. 



Now it has long been known that there exist in space, and 

 among the stars, vast cloudlike objects which are known as 

 nebulae. These are excessively tenuous, and are like gossamer 

 in texture ; indeed in many cases they are no more than a 

 shimmering veil through which the stars shine with un- 

 dimmed lustre. Some one has likened them to a breath- 

 stain on the window : Browning speaks of them as " figured 

 flames," and Tennyson describes them as "a fluid haze of 

 light." 



It was vaguely apprehended that this material might indeed 

 be the primitive star-plasm ; but so long as the telescope was 

 the only instrument of astronomical research, very little was 

 known regarding its real appearance and nature. I have 

 heard of a man who possessed what he considered to be an 

 exceptionally fine telescope, and who used to tell his friends 

 that when he looked through it at a distant church, the edifice 

 was brought so close to him that he could listen to the music 

 of the organ ! Needless to say, that man was ahead of his 

 time, for we have not yet succeeded in building a telescope 

 capable of achieving so remarkable a result. Nevertheless, 

 a large telescope is capable of accomplishing a great deal; 



