and Mechanical Condition on Radiant Heat. 305 



When A B was coated with lampblack and A' B' with rock- 

 salt powder, the equilibrium observed when both the plates w r ere 

 coated with lampblack did not exist. The lampblack, by its 

 greater absorption, heated its bismuth junction most, and a per- 

 manent 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 em- 

 ployed, 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 : — 



. Table VI. 



o 



Excess of lampblack above rock-salt . . .59=112 units. 



„ „ fluor-spar . . .46 = 68 „ 



„ „ red lead . . .40 = 45 „ 



„ „ oxide of cobalt . 37 = 42 „ 



„ „ sulphide of iron . 30 = 30 „ 



The order of absorption here shown coincides with the order 

 )f radiation of the same substances shown in Table III. But 

 vve 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 num- 

 bers given in Table VI. from 163, the total absorption of lamp- 

 black, we obtain a series of numbers which expresses the absorp- 

 tions of the other substances. This series stands as follows :— 





Table VII. 







Substance. 



Relative absorptions. 



, A . 



Radiation 



Rock-salt . . 



. . 51 



25-5 



25 



Fluor-spar . . 



. . 95 



47-5 



49 



Red lead . . 



. . 118 



59-0 



57 



Oxide of cobalt 



. . 121 



605 



62 



Sulphide of iron 



• . 133 



66-5 



66 



The first column of figures expresses the relative absorptions; 

 for the sake of comparison 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 



Phil. Mag. S. 4. Vol. 32. No. 216. Oct. 1866. X 



