OF RADIANT HEAT THROUGH DIFFERENT BODIES. 17 



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 v/hich they are emjjloyed. 

 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 M'hat 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 

 means 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, 

 Xve 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 it* 

 natural state before we make a second observation. Now it apjDcars 

 that the heat emitted by the flame peneti'ates 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 diffierence 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 



Vol. I — Part I. c 



