86 PEOPESSOK TYNDALL ON THE INPLTJENCE OF COLOUE 



pared a black surface with a white one; but the surfaces were seen to be white and 

 black through the transparent gum, which in both cases was the real radiator. The 

 same remark applies to Masson and Coubteepbb. Every particle 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 sub- 

 stance employed to make their powders cling to the surfaces of their cubes. 



Gum or glue is a powerful radiator ; in fact equal to lampblack, and it is a corre- 

 spondingly powerful absorber. The particles surrounded by it had therefore but small 

 chance of radiating through it. I sought to remedy this by the employment of a dia- 

 thermic cement. Sulphur is highly diathermic ; it dissolves 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 poiu'ed over the surface. Before the 

 liquid film had time to evaporate, the powder was shaken upon it through a muslin 

 sieve. The bisulphide passed rapidly away in vapour, leaving the powder behind 

 imbedded in the sulphur cement. Each powder, moreover, was laid on sufiiciently 

 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 improve- 

 ment on former methods, and yielded difierent 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 

 were heated with boiling water, and placed in succession at the same fixed distance in 

 front of the thermo-electric pile, which as usual was well defended from air-currents 

 and other extraneous sources of disturbance. Before giving the complete table of 

 results I will adduce a few of them, which show in a conclusive manner that in solid 

 bodies radiation is molecular rather than mechanical. 



The biniodide of mercury and the red oxide of lead resemble each other physically, 



both of them being of a brilliant red. Chemically, however, they are very different. 



Examined in the way indicated, their relative powers as radiators were foimd to be as 



follows: ISame. Chemical fommla. Radiation. 



Biniodide of Mercury . . (Hgl2) ^^'^ 



Red oxide of Lead . . . (2PbO, PbOg) 74-1 



Mixed with gum and applied with a camel's-hair brush to the surfaces of the cube, 

 the radiation from the following two substances fell out thus : — 



Name. Radiation. 



Binoxide of Mercury 80'0 



Red oxide of Lead 80-0 



Here the influence of the gum entirely masks the difference due to molecular consti- 

 tution. 



