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OF RADIANT HEAT THROUGH DIFFERENT BODIES. V7 
surface, the colour, and the internal structure of a body, as well as its 
chemical composition, have any influence whatever in the quantity of 
heat which it transmits immediately; and that, in this point of view, the 
origin and the qualities of the caloric rays become objects of perfect 
indifference; for it is enough for our purpose that the rays be invari- 
able and identical in all the circumstances in which they are employed. 
Now this actually is the case with the rays issuing from the well sup- 
ported flame of a Quinquet lamp placed at a fixed distance. 
When we shall have found the ratios of the quantities of heat trans 
mitted by screens of different kinds under the influence of a constant 
source, then, agreeably to what we have stated in the introduction, we 
shall examine the changes which those ratios undergo in consequence 
of the variation of the sources. 
All our experiments of comparison have been made with the same 
calorific radiation. Previously to the commencement of each series 
the rays were allowed to fall directly on the pile, and the distance of 
the lamp was modified until the needle of the galvanometer fixed itself: 
at 30° of the scale. 
’ We have remarked in the preliminary considerations, that all the 
external parts of the thermoscope are sheltered from the caloric rays by 
meaus of a large screen of polished metal, having in its central part a hole 
to correspond with the opening of the pile turned towards the lamp. 
In order to establish or to intercept the communication between this 
aperture and the source of heat securely and commodiously, we make 
use of a moveable copper screen, consisting of two or three parallel: 
plates fixed on the same support. The side of the pile opposite to the 
lamp may also be closed and opened by means of a screen altogether 
similar, and for the following purpose : 
~ When, after having observed the effect of any radiation whatsoever, 
we intercept the action of the source, we must wait until that face of 
the pile on which the rays of heat are darted has been restored to its 
natural state before we make a second observation. Now it appears 
that the heat emitted by the flame penetrates the apparatus with greater 
ease than it issues from it, because of its natural tendency to an equili- 
brium. At least the experiment shows that the time requisite to pro- 
duce the deviation is to that in which the needle recovers its original 
position nearly as one to five ; for the latter is from 7° to 8°, and we 
have seen that the whole deviation is produced in a minute and a half: 
Whatever be the cause of this difference between the time required for 
heating and that required for cooling, we must always allow 8° to elapse 
after one experiment before we proceed to another, if we confine our- 
selves to the placing of the first moveable screen before the radiating 
source. But let the opposite side of the pile be opened and a lighted 
candle brought close to the corresponding face: it is evident that if the 
Vor. l—Parr I. c 
