8 M. MELLOKI ON THE FREE TRANSMISSION 



with other plates of glass, or of any transparent substance whatsoever, 

 possessing different degrees of thickness, from the hundredth part of a 

 line to five or six inches, the galvanometer exhibits deviations greater 

 or less than 21°; but the time requisite to attain the equilibrium is in 

 all cases the same. In short, if we mark the time which the needle 

 takes to arrive at 30°, we shall find it to be one minute and a half. 



The invariability of this time, in such a vai'iety of circumstances, 

 affords the most decisive evidence that the deviations of the gal\ ano- 

 meter are exclusively due to that portion of heat which reaches the 

 pile by immediate transmission. Whence it follows, that in the arrange- 

 ment we have adopted, the heat of the transparent body has no appre- 

 tiable influence on the instrument. 



But a direct proof of this proposition may be obtained by operating 

 on opake screens. 



I take a plate of glass a millimetre in thickness. I blacken it on one 



uniform distribution over all the points of the massof air within, — a distribution 

 which will necessarily take place, because of the fluidity of the thermoscopic 

 body. 



Another inconvenience produced by the interposition of the glass, and from 

 which the thermomultiplier is free, is the lapse of a perceptible interval between 

 the commencement of the action and its manifestation on tlie instrument; for 

 there is always some time required, in order that the heat may pass from one 

 surface to tlie other. I speak not here of the caloric which might pass to the 

 air by free transmission through the diaphanous sides of the cover ; for when 

 we have to estimate the intensities of caloric rays by means of thennoscopes, we 

 cannot dispense with the blackening of the glass. So necessary indeed is this, 

 that in order to make sure of the opacity of the glass, it must be overlaid with 

 several coats of colouring matter. Otherwise, a portion of the rays would freely 

 pass through the mass of air contained in the ball without dilating it. 



Now, in the common thernioscopes, we always measure the radiation through 

 an opake plate of glass. This plate, however thin, must offer a considerable 

 resistance to the propagation of heat, because of the feebleness of its conducting 

 power, and will therefore, as we have already observed, render the apparatus 

 insensible during the first moments of action. Let it be observed, moreover, 

 that the more we endeavour to increase the sensibility of the thernioscope 

 by enlarging the dimensions of the balls, tlie more we diminish the promptitude 

 of its indications ; for the increase of vohmie is pi-oportionally greater than that 

 of the part of the surface turned towards the source, and the mass of air within 

 is increased in a proportionally greater degree than those points of the glass 

 which can communicate to it the heat they have acquired. Hence arises a 

 greater difficulty in attaining the moment of equal temperature in all the points 

 of the fluid mass, and, of course, the necessity of a longer time to mark the en- 

 tire effect. 



In fine, the thernioscopes are utterly useless when it is required to measure 

 caloric rays that are very feeble, and distributed according to given lines, or 

 forming sheaves of small dimension. In fact, it would be necessary in this case 

 to preserve the wholj sensibility of the instrument by considerably reducing 

 the size of the balls. But this is impossible. 



Whoever takes the trouble to weigh these considerations duly, will not, I 

 think, hesitate for a moment to prefer the theriiiomultiplier to every other ther- 

 moscopic apparatus in studying the subject of caloric radiation. 



