224 On the Temperature of Incandescence. 



From these considerations it seems to me we may fairly con- 

 clude, not that silicates, borates, phosphates and similar hyaline 

 compounds are absolutely non-luminous at very high tempera- 

 tures, but even though not many such compounds have been 

 carefully examined, that a substance which remains transparent 

 will be far less luminous at a high temperature than one which 

 becomes opaque. 



Sir J. F. ■ W. Herschel, in the text of his Outlines of 

 Astronomy, like many other observers, describes the physical 

 appearance of the sun as most nearly resembling a fused liquid 

 mass, covered by -a luminous envelope ; and while he argues for 

 a very high temperature in the sun, he suggested, 3 as long ago as 

 the year 1833, that it would be a highly curious subject of ex- 

 perimental enquiry, to determine how far a mere reduplication 

 of sheets of flame, one behind the other, would communicate to 

 the heat of the resulting compound ray, the penetrating charac- 

 ter, which distinguishes the solar calorific rays. Now two recent 

 investigators claim a low temperature in the sun, and the argu- 

 ment of one of them, Mr. W. M. Williams, is founded upon a 

 development of the idea thus advanced by Herschel in 1833, and 

 a study more or less reliable, of superposed radiant surfaces, 

 more especially of certain methods of heating iron plates, by ra- 

 diant heat from gas flames, of low temperature, but great body. 

 Many students of solar- physics remark the close resemblance of 

 the sun's surface phenomena to those presented by a fused flux 

 or slag, and its surface crust ; but the assumption that the sun 

 actually consists of a fused mass, covered by a pasty or solidifying 

 crust, has been opposed by four apparently inconsistent condi- 

 tions, namely : — first, its supposed enormously high temperature ; 

 — second, its supposed gaseous, because dark, interior; — third, 

 the supposed necessity that a more luminous crust should be at 

 a higher temperature than the fused material of which it is 

 formed; — and fourth, its low specific gravity. If the present 

 tendency to lower the temperature of the sun ends in the general 

 acceptance of figures even approximately as low as those of Dr. 



3 Herschel, J. F. W. Outlines of Astronomy; New York, 1859, page 212. Note from edi- 

 tion of 1833. 



