ON RADIANT IlEAT. 
337 
A Third Report [supplementary to two former Reports made to the 
British Association 1832 and 1840) on the present state of our 
knowledge of Radiant Heat. By the Rev. Baden Powell/JO,, 
F.R.S., F.R.A.S., F.G.S., Savilian Professor of Geometry in the 
University of Oxford. 
Part I. 
Having been honoured, for the third time, with a request from the General 
Committee to continue my former Reports on the state of our knowledge of 
Radiant Heat, from the date of ray last Report, 1840, up to the present time, I 
leel bound to explain that tho resolution conveying that request was passed 
so long since as the Meeting at York in 1851, ami that it was complied 
with on the understanding that the report was not required immediately. 
Rut at the present date, being unwilling longer to delay, and finding that 
trom the pressure of other avocatious there is little probability of my being 
able to complete it, rather than withdraw altogether, I run induced to iisk the 
indulgence of the Association for submitting only a portion of such a report. 
Pt'diminary remarks. 
Before commencing any analysis of recent investigations, it will he neces¬ 
sary (tor reasons which will become apparent) to take in the first instance a 
very brief retrospective glance at certain fundamental distinctions affecting 
nie whole subject, long ago pointed out, but too much overlooked by some 
writers since. It results from researches pointed out in detail in the two 
:7[ fi P orb - tha ^ the somewhat wide term “radiant heat” several 
\ r nct s P ec ‘ ea °f efl,cct have Been Included. 
U) file simplest form of radiant heat, and most properly so called, is that 
mch arises simply from the cooling of a hot body, and which emanates 
cwffu 8 , bodlc8 of ul1 temperatures, from those which are the least 
j ““ above V lat of the surrounding medium, up to the highest incan- 
cence °r combustion; and is distinguished by two properties : — 
RriTv 7, cn ?y t0 bc absor Bed by bodies in proportion to a certain pecu- 
(b\A *** Ul tbcir surface, but wholly independent of their colour, 
such a* t Utal inca P acit y to P ass by direct radiation through many media, 
throiml S ' &C '’ tbou ®“ transmissible freely through rock-salt and partially 
riments of C Mellon^ herS ’ CalI<><1 diathemanou ® media » as found by. the expe- 
J }'H a ^ crtain of incandescence other rays , also capable of exciting 
the DrISi 0 , e | lven ° p ffaloT ‘ff with the former-, these are distinguished by 
(5? ’ d,ffereDt from tl,e former:- 
a b s °rbeil by bodies in proportion to the darkness 
(3 F A ° Ur ° l 1 ' eir a b SOr P tio u of light. 
°ut tlitni P? wer ' °f transmissibility through all transparent, substances, with- 
ccilonrprf m a” tbrou 8b colourless media, and in various proportions through 
WBredmedia,according to their action on light. 
Tins Jp P awcr °f exciting also the sense of vision , or being luminiferous, 
most enninu fi ! pe ?J eS COe,rists in various Proportions with the first, and is 
III f t f , the " ,ost tensely ignited bodies, 
heating Jvt ^ a P a ' 0 = 0U9 to this last species, or identical with it, are the 
<*/•); and dta, ‘he wit; characterized by the same properties, (a.), (/3.) and 
kind (or Species I )» e< * ^ be,a ® totally free from all admixture of the first 
* Tbit I 
rtiermometer'to'th!! °* > * a ' ne ^ by exposing together a blackened and a white-washed 
1854. lar rayS ' first witb > ai,d then without, a thick glass screen: in this 
