24 



Mr. G. J. Stoney on the Physical 



[Recess, 



enormous temperature were it not for the escape of heat from it, which is 

 perpetually going on. The first and principal escape of heat takes place 

 from the photosphere, but it is also going on in the form of spectral lines, 

 whether visible or beyond the range of refrangibility that the eye can see, 

 from the upper layer of each gas that is successively left behind in ascending 

 through the atmosphere. The last escape of heat is from the hydrogen lines. 

 The stream of heat which passes per second through any spherical shell 

 concentric with the sun into those parts of the atmosphere that lie outside 

 it, is equal to what escapes per second from the latter into space. This 

 stream therefore remains constant wherever an interval exists between the 

 outer boundary of one gas and the bottom of that upper layer of the next 

 which is thick enough to be opake for the faintest of its spectral lines ; but 

 throughout the depth of each such upper stratum the stream of heat is on 

 the decrease. 



34. We shall better understand what takes place by considering the 

 agency by which the heat is carried outwards through the solar atmosphere. 

 It is partly by conduction, but principally by what may be called internal 

 radiation, to which are probably to be added in some situations convection 

 and irregular motions such as would result from storms. By conduction 

 I mean that conduction which is effected by the rectilinear motions of the 

 molecules. It is the only conduction to which experimentalists have found 

 it necessary to attend, since the quantities of transparent gas upon which 

 they operate are not such as to be, in the cool state in which they have 

 examined them, perceptibly opake to any of the incident rays. But when 

 the gas is incandescent and present in enormous quantity, the chief trans- 

 ference of heat through it will be in consequence of what I have called 

 internal radiation, which comes into play whenever the spectral rays emitted 

 by one part of the gas are absorbed by the surrounding parts before they 

 can reach the outer boundary and escape. If the gas be highly opake for 

 any particular ray, which is in general the case of those rays that appear 

 very bright in spectroscope experiments, it will travel but a short distance 

 before it is effectually absorbed ; but the rays which are faint in spectro- 

 scope experiments will wander further, and will contribute the most to the 

 rapid carriage of the heat to great distances. It should also be borne in 

 mind that if an extensive gas have a uniform temperature throughout, the 

 rays which at profound depths are dashing about, are all of the maximum 

 brightness corresponding to that temperature ; but that if the tempera- 

 ture of the gas be shaded off in one direction, as it is in the solar atmo- 

 sphere, the rays of internal radiation which are directed outwards at any 

 particular spot are brighter than the maximum brightness corresponding to 

 the temperature of that situation, since they come from warmer regions ; 

 and that those rays will be the brightest which in our experiments would 

 be faint, since they come from the most remote, and, therefore, from the 

 hottest of the parts from which any of the rays arrive. ^ 



35. It will not now appear strange that the region immediately outside 



