and Mechanical Condition on Radiant Heat. 295 



elusion we had the experiments of Melloni on chalk and lamp- 

 black, and the far more extensive ones of Masson and Courtepee 

 on powders, which seemed clearly to show that in a state of ex- 

 tremely fine division, as in chemical precipitates, the radiant and 

 absorbent powers of all bodies are the same. From these expe- 

 riments it was inferred that the influence of physical condition 

 was so predominant as to cause that of chemical constitution to 

 disappear*. 



A serious oversight, however, seems to have connected itself 

 with all the experiments of these distinguished men. Melloni 

 mixed his lampblack and powdered chalk with gum or glue, and 

 applied them by means of a cameFs-hair brush on the surfaces of 

 his radiating tube. Masson and Courtepee did the same. Mel- 

 loni, it is true, thus compared a black surface with a white one ; 

 but the surfaces were seen to be white and black through the trans- 

 parent gum, which in both cases was the real radiator. The 

 same remark applies to Masson and Courtepee. Every par- 

 ticle of the precipitates they employed was a varnished particle ; 

 and the constancy they observed was, I imagine, due to the fact 

 that the main radiant in all their experiments was the substance 

 employed to make their powders cling to the surfaces of their 

 cubes. 



Gum or glue is a powerful radiator — in fact, equal to lamp- 

 black ; and it is a correspondingly powerful absorber ; the par- 

 ticles surrounded by it had therefore but small chance of radia- 

 ting through it. I sought to remedy this by the employment 

 of a diathermic cement. Sulphur is highly diathermic ; it dis- 

 solves freely in bisulphide of carbon ; and at the suggestion of a 

 chemical friend I employed it to fix the powders. The cube 

 was laid upon its side, the surface to be coated being horizontal, 

 and the bisulphide, containing the sulphur in solution, was 

 poured over the surface. Before the liquid film had time to eva- 

 porate, the powder was shaken upon it through a muslin sieve. 

 The bisulphide passed rapidly away in vapour, leaving the pow- 

 der behind imbedded in the sulphur cement. Each powder, 

 moreover, was laid on sufficiently thick to prevent the sulphur 

 from surrounding its particles. This, though not perhaps a 

 perfect way of determining the radiation of powders, was at all 

 events an improvement on former methods, and yielded different 

 results. 



Ten or twelve cubes of tin were employed in the investigation. 

 One side of each of them was coated with milk of sulphur, and 

 this substance running through the entire series of cubes enabled 

 me to connect the results of all of them together. The cubes 



* Masson and Courtepee, Comples Rendus, vol. xxv. p. 938 ; Jamin, 

 Cours de Physique, vol. n. p. 289. 



