304 



Pi of. Tyndall on the Influence of Colour 



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 demonstra- 

 tion will, I trust, contribute to make inquirers unanimous on 

 this important point. 



Theory alone would lead us to the conclusion that the absorp- 

 tive power of the substances 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 contain- 

 ing 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 AB 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 dif- 

 ficult to make both junctions of absolutely the same sensitive- 

 ness ; 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 coat- 

 ings, 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. 



Stewart, of rock-salt radiating through rock-salt, is by itself sufficient to 

 demonstrate in the most unequivocal manner that this substance is not 

 equally pervious to all kinds of rays. 



