THE HISTORY OF A STAR. 75 



There are two or three points to consider with regard to the 

 history of a system, so long as it is on the rising part of the curve. 

 If we begin with globular condensations, such as those first de- 

 scribed by Sir William Herschel, we shall get, soon after the 

 initial stage, spiral and irregular intakes, and then these may in 

 time give place to rings such as we are already familiar with in 

 a member of our own system; I refer to the rings of Saturn. 

 Other dust-swarms near which such a system passes will be at- 

 tracted to it, and in addition to the initial revolving swarm and 

 its intakes and rings, we shall have a new order of things intro- 

 duced which we may term comets. 



Now the whole history of cometic astronomy goes to show that 

 no comet can enter such a system as ours without feeling the in- 

 fluence of the central system in a very remarkable way. We 

 know from other considerations that the nucleus of such a body 

 is simply a swarm of meteoritic dust-particles, large or small. 



The tail is always produced in a direction opposite to that of 

 the sun, and by some electrical energy, thermal energy, or what 

 not ; the result being that something is driven from the swarm of 

 meteorites in a direction away from the sun. Further, the stuff, 

 whatever it may be, thus repelled, is brought by the comet from 

 outer space ; for some of the short-period comets, those that never 

 leave our system, after they have passed round the sun a few 

 times, throw out no tail at all. 



If this can be universally proved for all comets, this is what 

 must happen : each central body will, by means of this energy, 

 place, as it were, a cordon round itself, inside of which no such 

 matter can remain as is thus driven off from comets and produces 

 the phenomena of a tail ; and if it be ever possible to state the 

 chemical nature of a comet's tail, the particular substances re- 

 pelled by this central energy will be known. It looks as if the 

 tails may consist, to a large extent, of the gases which exist in 

 meteorites, and which can be driven out of them at not very high 

 temperatures. Seeing that these are thrown off with great veloci- 

 ty and shine through millions of miles in the depths of space, it is 

 not likely that we are dealing with any such condensable sub- 

 stances as the vapors of iron, magnesium, or any other metal. 

 This consideration may help us eventually in the chemistry of 

 the repelling body. 



These revolving dust-swarms, as they increase their temper- 

 ature, will go through the same temperature changes as other 

 non-revolving ones. The existence of comets drawn into our sys- 

 tem from without, composed, like the nebula?, of meteoritic dust, 

 enables us to subject the view we are now considering to a very 

 crucial test. 



We know that the temperature of comets is increased, chiefly, 



