OXYGEN IN THE SUN. 13 



part Similarly, if hydrogen is introduced, it does not seek 

 the upper part of the vessel, but diffuses itself uniformly 

 throughout the vessel. If we enclose the carbonic acid gas 

 in a light silken covering, and the hydrogen hi another (at 

 the same pressure as the air in the vessel) one little balloon 

 will sink and the other will rise ; but this is simply because 

 diffusion is prevented. It may be asked how this agrees 

 with what I have said above, that some elements may not 

 exist in sufficient quantity or in suitable condition above the 

 sun's photospheric level to give any spectroscope evidence 

 of their nature. As to quantity, indeed, the answer is 

 obvious : if there is only a small quantity of any given element 

 in the entire mass of the sun, only a very small quantity can 

 under any circumstances exist outside the photosphere. As 

 regards condition, it must be remembered that the vessel of 

 my illustrative case was supposed to contain air at a given 

 temperature and pressure throughout If the vessel was so 

 large that in different parts of it the temperature and pressure 

 were different, the diffusion would, indeed, still be perfect, 

 because at all ordinary temperatures and pressures hydrogen 

 and carbonic acid gas remain gaseous. But if the vapour 

 introduced is of such a nature that at moderate temperatures 

 and pressures it condenses, wholly or in part, or liquefies, 

 the diffusion will not take place with the same uniformity. 

 We need not go further for illustration than to the case of 

 our own atmosphere as it actually exists. The vapour of 

 water spreads uniformly through each layer of the atmo- 

 sphere which is at such a temperature and pressure as to 

 permit of such diffusion ; but where the temperature is too 

 low for complete diffusion (at the actual pressure) the 

 aqueous vapour is condensed into visible cloud, diffusion 

 being checked at this point as at an impassable boundary. 

 In the case of the sun, as in the case of our own earth, it is 

 not the density of an element when in a vaporous form 

 which limits its diffusion, but the value of the temperature 

 at which its vapour at given pressure condenses into liquid 

 particles. It is in this way only that any separation can be 



