466 M. Melloni on the immediate Transmission 



thermal substance employed will then be applied against the 

 central opening of the metallic screen: it will immediately 

 transmit a certain quantity of radiant heat, which will pe- 

 netrate into the cylindrical covering of the pile placed at 

 a distance behind the screen, and directed upon the prolon- 

 gation of the line drawn from the source to the centre of the 

 opening: the indicating needle of the galvanometer con- 

 nected with the thermoelectric pile will be set in motion, and 

 will take a greater or a less deviation according to the dia- 

 thermaneity (diathermaneite) of the substance of which the 

 plate consists. After having noted this arc of deviation, let 

 the pile be removed by degrees from the direction of the im- 

 mediately transmitted calorific rays, taking care always to hold 

 the opening of its covering turned toward the plate, the di- 

 stance of which from the pile ought not to vary. We shall 

 then seethe deviation of the galvanometer diminish gradually, 

 and be reduced exactly to zero, when the covering of the pile 

 shall have entirely left the conical space occupied by the pencil 

 of emergent heat ; which supplies the most complete proof that 

 the heating effect due to the plate itself does not exercise the 

 least perceptible influence on the actual conditions of the 

 apparatus. 



To render the force of this demonstration still greater, 

 we may bring the pile several centimetres toward the plate, 

 while we remove it from the immediate direction of the rays. 

 We may also turn the plate upon its vertical axis, and place 

 it opposite the opening of the instrument removed from the 

 calorific cone, without the least deviation being manifested by 

 the galvanometer in either the one case or the other. 



It is thus decisively proved by this experiment, that the 

 heat from the source traverses the plate, preserving its radiant 

 form ; that the calorific rays are propagated beyond the plate 

 in their original direction only (dans le scul sens de leur direc- 

 tion primitive) ; and that all the effect produced, in the case in 

 which the axis of the pile is in front of the central opening of 

 the screen, is attributable to the action of the radiant heat 

 transmitted immediately by the plate. This mode of demon- 

 stration being independent of the nature of the rays, is equally 

 applicable to dark or luminous radiant heat. 



Now Mr. Hudson, in removing his thermo-electric pile out 

 of the direction of the calorific rays emitted by a vessel full of 

 hot water, finds that the needle of the galvanometer remains 

 at zero when the opening of the screen is free ; but he still 

 observes a very sensible deviation in the case in which the 

 opening is closed with the diaphanous plate. What must we 

 conclude? Evidentlv, that the circumstances under which 



