18 



Prof. J. Tyndall on 



[Dec. 18,. 



jacket W plays the part already assigned to it. The method of 

 experiment is now easy of comprehension. 



The whole space between C and t was, in the first place, rendered as 

 empty as a good air-pump could make it. The radiation from the 

 compensating cube C being cut off by the screen S, the total radiation 

 from C was declared by the galvanometer. The plate of salt was then 

 pushed up, and the consequent fall of the deflection was repeatedly 

 noted. Thus the absorption by rock-salt of radiant heat, which in no 

 part of its course had been sifted by air, was determined. Dry air 

 was then permitted to slowly enter the experimental tube until it was 

 filled. The length of air through which the rays had to pass before 

 reachiug the pile was ten times that in which Professor Buff had 

 found his large absorption. The sifting of the rays by the air, if 

 Professor Buff be correct, ought here to reduce the absorption of the 

 rock-salt to nothing. As in the former case, the experiment was 

 repeated many times, and on different days. There was no difference 

 between the transmission through the air and salt, and through the 

 vacuum and salt. The air proved absolutely powerless to influence in 

 any sensible degree the colour or quality of the heat. Here is a 

 sample of the results obtained : — 



Experimental Tube Exhausted. 



Badiation — 



o o 



Through vacuum, 41 '5 ; through salt, 37'5. 

 3? ?) 41*5 ; ,, ,, 37'5. 



J? 33 41 5 5 ,, ,, 37'5. 



41-5; „ „ 37-5. 

 The value of 41°'5is 51*5 units ; that of 37°'5 is 41 units — hence the 

 transmission amounts to 80 per cent, of the whole radiation. 



'Experimental Tube filled with Air. 



Badiation — 



Salt withdrawn, 39 5; through rock-salt, 35. 

 33 3? 39"5 ; ,, 35. 



33 3) 39"5 ; ,, „ 35. 



33 33 39'5; ,, „ 35. 



The value of 39°'5 is 46 units ; that of 35'°0 is 36*5 units— hence the 

 transmission amounts, as in the vacuum, to 80 per cent, of the whole 

 radiation. 



The experiment was checked, not only by repetition, but by the 

 following variation. 



By means of the adjusting screen S, the heat from the com- 

 pensating tube C was caused to accurately neutralise that from C y 



