32 



Mr. Gr. J. Stoney on the Physical 



[June 20, 



lines equally numerous, but faint. There is but one exception to this. 

 Hydrogen has a molecular mass so amazingly low (one twenty-third part 

 of the mass of molecules of sodium, the nearest to it in this respect of the 

 known constituents of stellar atmospheres), that there is probably no star 

 which can exert a force of gravity so powerful as to compel hydrogen to 

 limit itself to temperatures which show in any part of the spectrum a per- 

 ceptible degree of brightness when placed upon the background of the pho- 

 tosphere. In all stars accordingly in which hydrogen appears at all, the 

 four hydrogen lines are found intensely black. 



We see, then, why solitary stars are found of some particular colours only. 

 Stars which exert upon their outer atmospheres a force of gravity as great 

 or greater than the sun's are white : those on which gravity is a less force 

 are of some ruddy tint, — yellow, orange, or red. The foregoing results are 

 adjudged to be of probability 4, that is, fully made out. 



Those stars in which the force of gravity is very much less than on the 

 sun appear to form a distinct subclass. The four hydrogen lines are not 

 found in them, and at the same time new spectral lines, arranged in bands 

 each of which is closely ruled and fades off on the less refrangible side, 

 make their appearance. May we not here venture the suspicion that when 

 gravity upon a star is below a certain limit, such conditions prevail as com- 

 pel the hydrogen which would otherwise be free, to enter into combination 

 with some other element of low vapour-density ; and that the resulting 

 compound emits that spectrum of the First Order, as Pliicker has called it, 

 which we see ? 



To account for the colours of the companions of double stars we are again 

 forced to enter upon speculative ground. If the sky be peopled with count- 

 less multitudes of dark stars, which as well as the small number that are 

 Tisible, move only in virtue of their mutual attractions, it cannot be an ab- 

 solutely unusual occurrence for two stars to come into collision. Whenever 

 this happens, either the two stars emerge from the frightful conflagration 

 which would ensue as one star, or, if they succeed in disengaging them- 

 selves, they will be found after the catastrophe moving in new orbits. If 

 their previous courses had been parabolic, it can be shown that the new re- 

 lative orbit will be elliptic. Hence they will return to the charge again 

 and again, and at each perihelion passage there will be a fresh modification 

 of the orbit. It is shown that these modifications will in some instances 

 be such that the perihelion distance will be constantly on the increase, so 

 that the stars will, in their successive perihelion passages, climb as it were 

 asunder through one another's atmospheres. And the distance to which 

 they will ultimately withdraw before they separate will of necessity be im- 

 mense, since their atmospheres must have been dilated to a vast size by the 

 friction to which they have been subjected. As the stars recede from one 

 another the amount of heat which they generate at each perihelion passage 

 is progressively less and less, until at length the atmospheres of the stars 

 shrink in the intervals between two perihelion passages more than they ex- 



