i88 Mr. Powell’s experimental enquiry into the nature oj 
is produced on a blackened thermometer when a glass 
screen is interposed, in proportion as the body under trial 
approaches nearer its point of luminosity, or becomes more 
intensely luminous. (Biot, Traite de Phys. tom. iv. p. 638.) 
(Ann. of Philos. O. S. vol. ii. p. 163.) 
Both M. De La Roche and M. Biot (See Biot, iv. 612.) 
seem disposed to view the results obtained by the former 
upon the supposition of one simple agent, the principle both 
of light and heat. This is at first radiated as heat ; at a cer- 
tain point it begins to assume the form of light, when the 
interceptive power of glass decreases in proportion to the 
increase of luminosity. 
(3.) As long as the hot body continues below the tempera- 
ture of luminosity, the partial or total interception of the 
effect is precisely the same phenomenon as that described by 
Professor Leslie in his experiments on screens and explica- 
ble in the same way ; (Phil. Trans. 1816, Part I. On new 
Properties of Heat, Prop. 40.) And the apparent transmis- 
sion of a portion of the effect must be referred to the same 
principle, as is clearly shown by Dr. Brewster, who has 
established, apparently beyond contradiction, the imperme- 
ability of glass to simple radiant heat upon quite indepen- 
dent principles. 
(4.) Above the temperature of luminosity we must have 
recourse to further considerations. The hypothesis of M. M. 
De La Roche and Biot appears to be nearly the same as 
that of Professor Leslie (Inquiry, p. 162). And it certainly 
has the merit of simplicity and satisfactory explanation of 
the phenomena. But it is an opinion which has not received 
direct proof ; and it is also obvious, that the phenomena may 
