1887.] on the Sun's Heat 11 



space as much heat as all that it gets, both from the earth by contact, 

 and by radiation of heat from the earth, and by intercepted radiation 

 from the sun on its way to the earth. In the case of the sim the heat 

 radiated from the outer parts of the atmosphere is wholly derived 

 from the interior. In both cases the whole fluid mass is kept 

 thoroughly mixed by currents of cooled fluid coming down and 

 warmer fluid rising to take its place, and to be cooled and descend in 

 its turn. 



Kow it is a well known property of gases and of fluids generally 

 (except some special cases, as that of water within a few degrees of 

 its freezing temperature, in which the fluid under constant pressure 

 contracts with rise of temperature) that condensation and rarefactions, 

 effected by augmentations and diminutions of pressure from without, 

 produce elevations and lowerings of temperature in circumstances in 

 which the gas is prevented from either taking heat from or giving 

 heat to any material external to it. Thus a quantity of air or other 

 gas taken at ordinary temj)erature (say 15° C. or 59^ F.) and expanded 

 to double its bulk becomes 71° C. cooler; and if the expansion is 

 continued to thirty-two times its original bulk it becomes cooled 

 148° farther, or down to about 200° C. below the temperature of 

 freezing water, or to within 73° of absolute cold. Such changes as 

 these actually take place in masses^of air rising in the atmosphere to 

 heights of eight or nine kilometers, or of twenty or twenty-five 

 kilometers. Corresponding differences of temperature there certainly 

 are throughout the fluid mass of the sun, but of very different 

 magnitudes because of the twenty-seven fold greater gravity at the 

 sun's surface, the vastness of the space through which there is free 

 circulation of fluid, and last, though not least, the enormously higher 

 temperature of the solar fluid than of the terrestrial atmosphere at 

 points of equal density in the two. This view of the solar constitution 

 has been treated mathematically with great power by Mr. J. Homer 

 Lane, of Washington, U.S., in a very important paper read before the 

 National Academy of Sciences, of the XJnited States in April 1869, 

 and published with further developments in the ' American Journal of 

 Science,' for July 1870. Mr. Lane, by strict mathematical treatment 

 finds the law of distribution of density and temperature all through a 

 globe of homogeneous gas left to itself in space, and losing heat by 

 radiation outwards so slowly that the heat-carrying currents produce 

 but little disturbance from the globular form. 



One very remarkable and important result which he finds is, that 

 the density at the centre is about twenty * times the mean density ; 

 and this, whether the mass be large or small, and whether of oxygen, 

 nitrogen, or hydrogen, or other substance ; provided only it be of 

 one kind of gas throughout, and that the density in the central parts 

 is not too great to allow the condensation to take place, according to 



♦ "Working out Lane's problem independently, I find 22^ as very nearly the 

 exact number. 



