84 PEOFESSOE TYNDALL ON THE INFLTJENCE OF COLOUE 



that of its neighbour. Two sheets of tin were coated, the one with alum, and the other 

 with iodine powder. The sheets were placed parallel to each other and about 10 inches 

 asunder ; at the back of each was soldered a little bar of bismuth, which with the tin 

 plate to which it was attached constituted a thermo-electric couple. The two plates 

 were connected together by a wire, and the free ends of the bismuth bars were con- 

 nected 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 galvanometer imme- 

 diately 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 mushn sieve ; 

 in other cases it was mixed with bisulphide of carbon and applied with a camel's-hair 

 brush. When dried afterwards it was almost as black as soot, but as an absorber of 

 radiant heat it was no match for the perfectly white powder of alum. 



The difiiculty 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 

 reflexion. 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 con- 

 siderable 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 phosphorus. A small fragment of this exceedingly inflam- 

 mable substance 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 phos- 

 phorus 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 phosphorus soon fuses and ignites ; its ignition, 

 however, is not entirely due to radiant heat, but mainly to the heat imparted to it by 

 the glass*. 



The fusing-point of phosphorus is about 44° C, that of sugar is 160°; still at the focus 

 of the electric lamp the sugar fuses before the phosphorus. All this is due to the dia- 



* I believe this deportment of phosphorus towards radiant heat is not unknown to chemists. 



