260 On the Bjundary of a IVace ofCunducled Heat. 



The remai-kable cliaiige of colour wliicli lieat produces in tliis 

 iodide led me to hope that this molecular change would be ac- 

 companied bv a simultaneous variation in its radiating- power. 

 To solve this problem I made the following experiments at dif- 

 ferent temperatures, below and above the degree at which its 

 colour chan2:es. One side of a hot-water cube was coated with 

 lampblackj and another side with a thick paste of iodide and 

 gum-water; after the latter had nearly dried I sifted iodide over 

 it, and caused this to adhere by rubbmg it gently with my finger. 

 The cube was now filled with water, in which was supported a 

 thermometer. The water was raised to the following tempera- 

 tures, and frequently agitated so as to ensure a uniform heating 

 of the cube. The dedections produced in the galvanometer- 

 needles bv the lampblack and by the iodide were then obtained 

 for each fixed temperature. Each deflection given below is the 

 mean of three experiments. 



Temp. Lampblack. Iodide. Ratio of deflections. Changes in colour. 



a 



70 



^ r Cherry-red, and turning 

 ' \in spots to chocolate colour. 

 .^ J' Dark red, with spots of 

 ' . chocolate colour. 



60 



18-75 



13-75 



1 



65 



22-25 



17 



1 



68 



22-75 



16-25 



1 



70 



24-0 



16-87 



1 



72 



25-0 



17-62 



1 



75 



26-25 



18-62 



1 



100 



45-0 



30-5 



1 



70 < 



f 'Whole surface of a deep 



brown. 

 '70 Deep purplish brown. 

 '^0 „ „ 



■67 



The last experiment, in which the temperature of the surface 

 was 100^, gave deflections so far exceeding those produced before, 

 that I sought to render them comparable by removing the hot- 

 water cube to a greater distance from the thermobattery, when I 

 obtained the following ratio : — 



Temp. Lampblack. Iodide. Ratio. 



100' 20' 13-11 1 : -Qr 



The result was the same ratio as formerly obtained. 



These experiments seem to show that the molecular change in 

 the iodide, which causes it to act so diff'erently in reflecting light, 

 does not appear to have any action on its power of radiating the 

 rays of heat of low intensity. I intend, however, to return to 

 this investigation, provided with an apparatus giving the differ- 

 ential actions of two cubes and having a carefully calibrated 

 galvanometer, and with this arrangement to test the reflecting- 

 as well as the radiatiug-po-ver of this and other iodides. 



Several applications of this iodide for showing elevations of 



