404 THE CHEMISTRY OF THE SUN. [CHAP. 



different orders of density. The same thing happens with 

 regard to gases. We know that hydrogen is less dense than 

 oxygen arid nitrogen, and so on. 



Now, what is the density of the sun ? Is the sun denser 

 than the earth ? No ; according to the books it is just about a 

 quarter as dense as the earth, so that it is a little denser than 

 water. In fact, if we take water as our unit of density, the 

 density of the sun is 1'444. If we take the density of the earth 

 as one, then the value for the sun is about O25. 



These are the values given in the books, but they have been 

 determined by taking the volume of the sun as given by the 

 diameter of the photosphere 860,000 miles. But if we con- 

 cede 100,000 miles for the height of the layers of atmosphere 

 above the photosphere, those layers must not be left out of 

 consideration. If we include these layers, though we do not alter 

 the mass, we alter the volume. If we put the same mass into a 

 larger volume, we naturally reduce the density. Now, if we 

 take the atmosphere of the sun as extending to 100,000 miles 

 above the photosphere, that will give us a radius of 530,000 

 miles, instead of 430,000 miles, and we shall, as nearly as may 

 be, double the sun's volume. Therefore we shall have halved 

 the density. Instead of being a quarter as dense as the earth, 

 it will only be one-eighth as dense ; and, instead of being just 

 denser than water, it will be a little over half the density of 

 water. For my own part, I think that this 100,000 miles is not 

 sufficient. I think that it is the minimum, and that most 

 students of solar physics would agree that a height for this 

 purpose of 500,000 miles above the photosphere would be 

 probably nearer the mark. That will give us exactly ten times 

 the volume of the sun bounded by the photosphere, so that the 

 mean density will be reduced to the tenth ; we shall get a mean 

 density then of about one-eighth that of water. This, of course, 

 is the average density of the whole volume in which the mass 

 is supposed to be diffused the mass which is a fact which we 



