AND MECHANICAL CONDITION ON BADIANT HEAT. 



95 



Table V. 

 Radiation through Eock-salt. 



Source. Absorption. 



Black Platinum 3 "7 



Black oxide of Iron 13-0 



Red oxide of Iron 15'9 



Sugar 17'3 



Chloride of Silver 22-6 



Rock-salt 29-9 



These differences of absorption are so great as to enable every experimenter to satisfy 

 himself with the utmost ease as to the unequal permeability of rock-salt, and this facility 

 of demonstration will, I trust, contribute to make inquirers unanimous on this important 

 point. 



Theory alone would lead us to the conclusion that the absorptive power of the sub- 

 stances mentioned in Table I. is proportional to their radiative power ; nevertheless a 

 few actual experiments on absorption will serve as a check upon those recorded in the 

 Table. These were conducted in the following manner: — 

 AB is a sheet of common block tin, 5 inches high by 4 

 in width, fixed upon a suitable stand. At the back of A B 

 is soldered one end of the small bar of bismuth b, the 

 remainder of the bar, to its free end, being kept out of contact 

 with the plate by a bit of cardboard. To the free end of b is 

 soldered a wire which can be connected with a galvanometer. 

 A' B' is a second plate of metal in every respect similar to A B. 

 From one plate to the other stretches the wire W. C is a 

 cube containing boiling water, placed midway between the 

 two plates of metal. 



The plates were- in the first instance coated uniformly with lampblack, and the two 

 surfaces of the cube which radiated against the plates were similarly coated. The rays 

 from C being emitted equally right and left, and absorbed equally by the two coated 

 plates A B and A' B', warmed these plates to the same degree ; it is manifest from the 

 arrangement that, if the thermo-electric junctions were equally sensitive, the current 

 generated at the one ought exactly to neutralize the current from the other junction. 

 This was found to be very nearly the case. It is difficult to make both junctions of 

 absolutely the same sensitiveness ; but the moving of the feebler plate a hair's breadth 

 nearer to the cube C enabled it to neutralize exactly the radiation from its opposite 

 neighbour. My object now was to compare the lampblack coating of the plate A B with 

 a series of other coatings, which were placed in succession on the other plate. These 

 latter coatings were the powders already employed, and they were held upon A' B' by 

 their own adhesion. 



When A B was coated with lampblack and A' B' with rock-salt powder, the equili- 



