TRANSACTIONS OF THE SECTIONS. 381 



vations were made on different days and at different hours of 

 the same day. Professor Forbes has not yet finally reduced 

 his observations ; but from the approximate numbers communi- 

 cated by Sir John Herschel to the Meeting, it appears that even 

 in the clearest weather (in which alone these experiments were 

 made,) the force of solar radiation is diminished by about one 

 fifth in passing through a column of 6000 feet of the purest 

 air. 



Account of some recent Experiments on Radiant Heat. By 

 M. Melloni. Communicated by Professor Forbes. 



The following truths at which M. Melloni has arrived are of 

 capital importance. The quantity of calorific rays which tra- 

 verse a screen, derived from sources of high or low tempera- 

 ture, is proportional to that temperature ; but the difference 

 constantly diminishes as the thickness of the screen becomes 

 less, till at last, with excessively thin lamince of mica, it is inseti- 

 sible. This remarkable result proves that the resistance to 

 passage which affects the transmission of heat depending on 

 the temperature of its source, is not exerted at the surface, but 

 in the interior of the mass. 



The extension of this observation to the solar rays gives the 

 following result. M. Melloni operated with various thicknesses 

 of sulphate of lime, water, and acids. He found that the quan- 

 tity of heat intercepted by increasing the thickness of the trans- 

 parent medium is greater for the less refrangible than for the 

 more refrangible rays ; that is, that while for example an 

 evanescent pellicle of mica would permit as much per cent, of 

 the heat accompanying the red ray to pass through it as of the 

 violet ray, if the thickness be increased, a much larger per- 

 centage of the former will be stopped than of the latter; whence 

 the author concluded that the refrangibility of a heating ray is 

 coordinate with, and a measure of, its intensity. Thus, a plate 

 of glass S"""" in thickness stops, out of 100 incident rays of heat, 

 45 when they proceed from a lighted candle, 70 from a mass of 

 copper at 950° Cent., 92 from a vessel of boiling mercury, and 

 100 from a vessel of boiling water. 



Hence the impossibility, as he observes, by the use of lenses 

 of glass and suchlike materials of concentrating heat of low 

 temperature. This admits, however, of one most important 

 exception, the discovery of which forms an epoch in the history 

 of the science of heat. In a previous memoir, M. Melloni had 

 compared the transmissive powers of a great number of sub- 

 stances. For uncrystallized compounds and fluids, he found 



