96 PEOFESSOE TTNDALL ON EADIATION AND ABSOEPTION. 



brium observed when both the plates were coated with lampblack did not exist. The 

 lampblack, by its greater absorption, heated its bismuth junction most, and a perma- 

 nent deflection of 59° in favour of the lampblack was obtained. Other powders were 

 then substituted for the rock-salt, and the difference between them and the lampblack 

 was determined in the same way. When, for example, sulphide of iron was employed, 

 there was a deflection of 30° in favour of lampblack. The results obtained with six 

 different powders thus compared with lampblack are given in the following Table : — 



59 = 112 units. 



46 = 68 „ 

 40= 45 „ 

 37= 42 „ 

 30 = 30 „ 



Table VI. 



Excess of lampblack above Rock-salt . . . 

 „ „ Fluor-spar . . 



„ „ Red Lead . 



„ „ Oxide of Cobalt . 



„ „ Sulphide of Iron 



The order of absorption here shown coincides with the order of radiation of the 

 same substances shown in Table III, But we can go further than the mere order of 

 absorption. Removing the opposing plate, and allowing the standard lampblack to 

 exert its full action upon the galvanometer, the deflection observed was 



65°= 163 units. 



The numbers in Table VI. show us the excess of the lampblack over the substances 

 there employed; its excess in the case of rock-salt, a bad absorber, being 112, its excess 

 in the case of sulphide of iron being only 30. Deducting, therefore, the numbers 

 given in Table VI. from 163, the total absorption of lampblack, we obtain a series of 

 numbers which expresses the absorptions of the other substances. This series stands as 

 follows : — 



Table VII. 



The first column of figures expresses the relative absorptions; for the sake of com- 

 parison with the corresponding radiations, I have placed the halves of these numbers in 

 the second column of figures, and in the third column the radiations obtained from 

 Table II. The approximation of the figures in the second and third columns is seen to 

 be extremely close. 



Throughout this investigation I have been efficiently assisted by Mr. W. F. Barrett, 

 whose rapid progress in scientific knowledge and experimental skill during the three 

 years that he has assisted me has given me great satisfaction. 



