310 Royal Society :— Mr. G. J. Stoney on the Physical 



epoch — just as our November meteors were brought in by the planet 

 TJranus in the year 126 of the Christian era. 



Finally it is shown that an hypothesis which has found much 

 and deserved favour of late years, that the heat expended by the sun 

 is continually restored to him by the falling in of meteors which 

 had been circulating round him, is no longer tenable. 



The second part of the memoir treats of other stars. The differ- 

 ences in their appearances are found to depend mainly on differences 

 in the force of gravity exerted at their surfaces. Where gravity on a 

 star is feebler than on the sun, either from the mass of the star being 

 less, or from its being so dilated by heat that its outer parts are 

 further removed from its centre, gases which by reason of the mass 

 of their molecules are imprisoned within the photosphere of the sun, 

 will, when less attracted downwards, be able to stand the coolness of 

 the shell of clouds and pass beyond them. Thus mercury, anti- 

 mony, tellurium, and bismuth, all of which have too high a vapour- 

 density to exist in the sun's outer atmosphere, show themselves in 

 that of Aldebaran. Again, in these stars all the gases of the outer 

 atmosphere expand until their upper layers, those from which their 

 spectral lines issue, are cooler than on the sun. These spectral lines 

 will accordingly be darker than on the sun, and as this will tell with 

 most effect on the blue end of the spectrum, it will render the light 

 from these stars ruddy. 



On the other hand, those stars which, either from being of greater 

 mass than the sun, or from being less hot in their internal parts, at- 

 tract down the gases of their outer atmospheres with more force, 

 constitute the class of intensely white stars with a somewhat violet 

 tinge, of which Sirius and a Lyrse are examples. Several of the 

 substances which in the sun's spectrum give rise to faint lines, are 

 on such stars confined within the photosphere ; and the lowest 

 temperature which others of them can withstand is, by reason of the 

 force with which they are attracted downwards, hotter than the cor- 

 responding temperatures of the sun. Hence the substances which 

 on the sun cause his numerous dark lines — sodium, magnesium, cal- 

 cium, chromium, manganese, iron — produce, in the spectrum of the 

 star, lines equally numerous, but faint. There is but one excep- 

 tion 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 perceptible degree of 

 brightness when placed upon the background of the photosphere. In 

 all stars accordingly in which hydrogen appears at all, the four hy- 

 drogen 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. 



