1815.] of the Effect of Ice on a Thermometer. 341 



the distance of one foot from the thermon:ieter, and the wall of the 

 room be at tlie distance of eight feet from the same, the screen 

 intercepts from the thermometer an area of 2;i04 square inches ; 

 but the intensity of the radiation from tiie wall, which is at eight 

 times the distance of the screen, is to the intensity of tlie radiation 

 from the screen only as one to 64, or as 36 to 2;-504; and thus the 

 difference in quantity of radiating surface is compensated by the 

 difference in intensity. 



Now by mere reflection the actual number of rays, or the inten- 

 sity of any given number, caimot be either increased or diminished. 

 The intensity (whether of light or heat) is increased in the focus 

 solely by the direction to one poiiit of a number of rays subtracted 

 by a cliange of direction from other points on which they would 

 have fallen in their natural course ; for it is self-evident that what- 

 ever radiation is superadded by mere reflection on one spot, must 

 be subtracted from some other ; and if the sensible heat is increased 

 in the focus, it must be diminished somewhere «lse. 



Some kinds of surfaces receive and emit heat by radiation with 

 great facility ; otiiers reflect more and radiate less ; as Mr. Leslie 

 has shown. Mr. Leslie has also shown that the powers of receiving 

 and giving out heat are in the same surface equal ; and also that its 

 powers of radiatic«i and reflection are in inverse ratios. This seemi 

 also demonstrable from all usual appearances; for if any body 

 ci)uld radiate eitlier more or less than it receives, or reflecting a part 

 only of what falls on it, did not give out by radiation a sum equal 

 to that which enters, not only would adjacent bodies be affected by 

 it, but the sensible heat of the lM»dy itself must contitiue to increase 

 or diminish, without apparent limits : but it is not found that the 

 temperatures of adjacent or remote bodies differ from each other by 

 mere position, where there is no source of lieat or active cause of 

 increase or diminution of the actual quantity of free caloric ; and 

 since different surfaces of the same body radiate op reflect one more 

 than another, if the emission and int«3mission by radiation were 

 not equal in the same surface, or if the reflecting surface did not 

 radiate all which it does not reflect, bodies exposed near one surface 

 must be affected differently from those on the other. Thus if a flat 

 plate of metal were |)olished on one side, a-nd blackened on the 

 other, two bodies of equal temperature with the plate must receive 

 «Jirterent temperatures from it ; niiW more would a body in the prin- 

 cipai focus of a reflecting mirror be affected ; for there is an 

 actual accumulation of reflected rays upon it. Yet a body placed 

 in or oBt of the focus, or even before the concave or behind the 

 conven ade of the mirror, suffers neither increase nor decrease of 

 temperature, neither is it affected by the proximity or removal of 

 either tlw; plane retiector or the mirror. Where is the compensation? 

 In the case of the j)lane reflecting plate, the radiation of nearly 

 one half of the surrounding sphere is intercepted, viz. all behind 

 the plate ; but the same quantity of radiation from the hemisphere 

 in front of tlic plate which would have (which has, indeed,) passed 



