38 SECTIONAL ADDRESSES. 



the star on the other hand. \Ye can thus find certain information as to 

 the inner material, as though we had actually bored a hole. So far as 

 can be judged there are only two physical properties of the material 

 which can concern us — always provided that it is sufficiently rarefied 

 to behave as a perfect gas — ^viz. the average molecular weight and 

 the transparency or permeability to radiant energy. In connecting 

 these two unknowns with the quantities given directly by astronomical 

 observation we depend entirely on the well-tried principles of conserva- 

 tion of momentum and the second law of thermodynamics. If any 

 element of speculation remains in this method of investigation, I think 

 it is no more than is inseparable from every kind of theoretical advance. 



We have, then, on the one side the mass, density and output of 

 heat, quantities as to which we have obserA'^ational knowledge; on the 

 other side, molecular weight and transparency, quantities which we 

 want to discover. 



To find the transparency of stellar material to the radiation 

 traversing it is of particular interest because it links on this 

 astronomical inquiry to physical investigations now being carried on in 

 the laboratory, and to some extent it extends those investigations to 

 conditions unattainable on the earth. At high tempei'atures the 

 ether waves are mainly of very short wave-length, and in the stars we 

 are dealing mainly with radiation of wave-length 3 to 30 Angstrom 

 units, which might be described as very soft 3"-rays. It is interesting, 

 therefore, to compare the results with the absorption of the harder 

 x-rajs dealt with by physicists. To obtain an exact measure of this 

 absorption in the stars we have to assume a value of the molecular 

 weight; but fortunately the extreme range possible for the molecular 

 weight gives fairly narrow limits for the absorption. The average 

 weight of the ultimate independent particles in a star is probably 

 rather low, because in the conditions prevailing there the atoms would 

 be strongly ionised ; that is to say, many of tlie outer electrons of the 

 system of the atom would be broken off; and as each of these free 

 electrons counts as an indejjendent molecule for the present purposes, 

 this brings down the average weight. In the extreme case (probably 

 not reached in a star) when the whole of the electrons outside the 

 nucleus are detached the averagte weight comes down to about 2, 

 whatever the material, because the number of electrons is about half 

 the atomic weight for all the elements (except hydrogen). We may, 

 then, safely take 2 as the extreme lower limit. For an upper limit we 

 might perhaps take 200 ; but to avoid controversy we shall be generous 

 and merely assume that the molecular weight is not greater than — 

 infinity. Here is the result: — 



For molecular weight 2, mass-coafficient of absorption = 10 



C.G.S. units. 

 For molecular weight oo , mass-coefficient of absorption = 130 

 C.G.S. units. 

 The true value, then, must be between 10 and 130. Partly from 

 thermodynamical considerations, and partly from further comparisons 

 of astronomical observation with theory, the most likely value seems 

 to be about 35 C.G.S. units, corresponding to molecular weight 3*5. 



