O M. MELLONI ON THE I'UEE TRANSMISSION 



If between the flame of a candle antl the eye we interpose a plate of 

 glass or any other substance more or less transjiarent, we find tJie di- 

 minution of the intensity of the light always the same, however the di- 

 stance between the plate and the candle may vary. The effect produced 

 by distance on the freely transmitted caloric is exactly similar ; and if 

 at a certain distance from the active source there be a thermoscopic 

 apparatus sensible to this portion of heat, the apparatus will always give 

 the same indication, whether the screen be laid close to the source or 

 to the thermoscope. 



But it is clear that it must happen quite otherwise to the conductible 

 caloric ; for this portion of the heat, when it has reached the further sur- 

 face of the screen, leaves it in the form of diverging rays which become 

 weaker in proportion to the distance. In other words, the further sur- 

 face of the screen being heated becomes a new calorific source whose 

 intensity of radiation must decrease as the distance increases. 



We possess, therefore, a verj' simple contrivance for destroying the 

 influence of conduction, if we keep the action of the free radiation in- 

 tact. This contrivance consists in removing the screen so far from the 

 thermoscope that the radiation of its own heat may, on account of its 

 extreme feebleness, be totally disregarded. 



There are, however, some precautions to be taken ; for in proportion 

 as the distance between the screen and the thermoscope is increased, 

 the distance between the source and the screen is diminished. The 

 latter is therefore more heated, and radiates with greater force upon 

 the instrument. It is easy to show by calculation that we always gain ; 

 that is, that we always weaken the conductible caloric more and more 

 by removing the screen from the thermoscope, until we have placed it 

 midway between the thermoscope and the source*. Let us, therefore, 

 put the screen in this position (which is the most favourable of all), and 

 we shall see that its heat has then no appretiable influence on the i-e- 



* Let a be the distance from the source to the thermoscope, x the distance 

 from the thermoscope to the screen, i the calorific intensity of the source, we 



i 

 shall have . .^ as the expression for the radiation which strikes the anterior 



Kiuface of the screen. This quantity will become , -2 ^^ 'he further sur- 

 face, c being a constant quantity depending on the conducting power of the 

 matter of the screen. In fine, the radiation of the further surface on the ther- 



moscope will be expressed by — r tj ; its minimum (y) is to be determined. 



XT 1 Trt- ,■ • ■, . dy 2ci(2x — a) , . , . , 



Now, by difierentiatmg we obtam -r^^ = 3 . _ ,3 ; the equation which gives 



the quantity will then be 2x — a=zo, whence .r= .,. 



