VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 215 



duum of 39grs., and a little more; in the residuum left by equal quantities of 

 iron, before and after the experiment in the unglazed tube, there was a difference 

 of 5 grs. ; the solution of the iron that had been submitted to the experiment 

 went on very slowly ; and would not have been effected by vitriolic acid in many 

 months. In the latter case I used some muriatic acid, which quickly dissolved it: 

 in the former, weak aqua regia was used for the solution of a very small part of 

 the whole lump. I suspected lead to have caused the slowness of the solution in 

 the first case, but there can be no such suspicion in the 2d. The difference 

 between these residuums tends to show that plumbago was consumed by the heat : 

 but they do not show the loss accurately ; for in the residuum of the iron that 

 had been fused in the first experiment, there was a small quantity of vitriolated 

 lead ; and in the other there was, besides the plumbago, a small quantity of that 

 difficultly soluble calx of iron, which the solutions of this metal deposit on long 

 exposure to air. The difference was greater therefore than it appeared. On the 

 other hand, the long action of the acids might have consumed some plumbago. 

 There was little or no calx attractable by the magnet in the residuums of the 

 fused iron. From the 48 grs. of residuum, I separated more than 6 grs. by the 

 magnet. 



XI II. Continuation of a Paper on the . Production of Light and Heat from 

 different Bodies. By Mr, Thomas Wedgwood, p. 2/0. 



Exper. \. In order to discover what effect the light of the burning fuel has 

 on incombustible bodies, says Mr. W., I fixed into the end of a tube of earthen- 

 ware* 2 equal cylinders of silver, with polished surfaces, half an inch in length, 

 and a quarter of an inch in diameter, see pi. 2, fig. 6 ; one of the cylinders was 

 painted over, except the end within the tube, with a thin coat of incombustible 

 black colour, to make it absorb the incident light ; the other, intended to reflect, 

 was left with its polished surface. Applying my eye to the opposite extremity 

 of the tube (which is fitted exactly, so that no extraneous light could enter), 

 and directing it towards the 2 polished ends of the cylinders, I held the tube 

 within a red-hot crucible, surrounded by burning coaks, and continually turned 

 it round, that both cylinders might be equally exposed to the light and 

 heat. The result was, that the end of the blackened cylinder began to shine a 

 considerable time before that of the polished one, and remained constantly 

 somewhat brighter: on removing the tube from the crucible, still looking within 

 it, I was surprized to see the appearance reversed, the polished cylinder continu- 

 ing to shine for some time after the blackened one had ceased. Cylinders of 

 gold, and of iron, treated in the same manner, gave the same general result; 



* When earthea-ware is mentioned in this paper, the cream-coloured or queen's ware is meant. — 

 Orig. 



