Prof. Tyndall on Radiation and Absorption. 293 



soldered a little bar of bismuth, which with the tin plate to 

 which it was attaclied constituted a thermo-electric couple. The 

 two plates were connected together by a wire, and the free ends 

 of the bismuth bars were connected with a galvanometer. Placing 

 a red-hot ball midway between both, the calorific rays fell with 

 the same intensity on the two sheets of tin, but the galvano- 

 meter immediately declared that the sheet which bore the alum 

 was the most highly heated. 



In some of the foregoing cases the iodine was simply shaken 

 through a muslin sieve ; in other cases it was mixed with bisul- 

 phide of carbon and applied with a camePs-hair brush. When 

 dried afterwards it was almost as black as soot ; but as an ab- 

 sorber of radiant heat it was no match for the perfectly white 

 powder of alum. 



The difficulty of warming iodine by radiant heat is evidently due 

 to the diathermic property which it manifests so strikingly when 

 dissolved in bisulphide of carbon. The heat enters the powder, is 

 reflected at the limiting surfaces of the particles, but it does not 

 lodge itself among the atoms of the iodine. When shaken in 

 sufficient quantity on a plate of rock-salt and placed in the path 

 of a calorific beam, iodine cuts the latter off. But its opacity is 

 mainly that of a white powder to light; it is impervious, not 

 through absorption, but through internal reflexion. Ordinary 

 roll sulphur, even in thin cakes, allows no radiant heat to pass 

 through it; but its opacity is also due to repeated internal re- 

 flexion. The temperature of ignition of sulphur is about 244° 

 C. ; but on placing a small piece of the substance at the focus 

 of the electric lamp, where the temperature was sufficient to heat 

 platinum-foil in a moment to whiteness, it required exposure for 

 a considerable time to fuse and ignite the sulphur. Though 

 impervious to the heat, it was not adiathermic. The milk of 

 sulphur was also ignited with some difficulty. Sugar is a much 

 less inflammable substance than sulphur, but it is a far better 

 absorber ; exposed at the focus, it is speedily fused and burnt 

 up. The heat moreover which is competent to inflame sugar is 

 scarcely competent to warm table-salt. 



A fragment of almost black amorphous phosphorus was exposed 

 at the dark focus of the electric lamp, but refused to be ignited. 

 A still more remarkable result was obtained with ordinary phos- 

 phorus. A small fragment of this exceedingly inflammable sub- 

 stance could be exposed for twenty seconds without ignition at 

 a focus where platinum was almost instantaneously raised to a 

 white heat. Placing a morsel of phosphorus on a plate of rock- 

 salt and holding it before a glowing fire, it bears, as proved by 

 my assistant, Mr. Barrett, an intense radiation without ignition ; 

 but laid upon a plate of glass and similarly exposed, the phos- 



