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XXXVIII. Sixth Memoir on Radiation and Absorption — Influ- 

 ence of Colour and Mechanical Condition on Radiant Heat. By 

 Professor J. Tyndall, LL.D., F.R.S., %c .* 



FRANKLIN placed cloths of various colours upon snow and 

 allowed the sun to shine upon them. They absorbed the 

 solar rays in different degrees, became differently heated, and 

 sank therefore to different depths in the snow beneath them. 

 His conclusion was that dark colours were the best absorbers, 

 and light colours the worst ; and to this hour we appear to have 

 been content to accept Franklin's generalization without qualifi- 

 cation. In my last memoir I briefly pointed out its probable 

 defects. Did the emission from luminous sources consist exclu- 

 sively of visible rays, we might fairly infer from the colour of a 

 substance its capacity to absorb the heat of such sources. But 

 the emission from luminous sources is by no means all visible. 

 In terrestrial sources by far the greater part, and in the case of 

 the sun a very great part, of the emission consists of invisible 

 rays, regarding which colour teaches us nothing. 



It remained therefore to examine whether the results of 

 Franklin were the expression of a law of nature. Two cards 

 were taken of the same size and texture ; over one of them was 

 shaken the white powder of alum, and over the other the dark 

 powder of iodine. Placed before a glowing fire and permitted 

 to assume the maximum temperature due to their position, it 

 was found that the card bearing the alum became extremely 

 hot, while that bearing the iodine remained cool. No thermo- 

 meter was necessary to demonstrate this difference. Placing, 

 for example, the back of the iodine card against the forehead or 

 cheek, no inconvenience was experienced ; while the back of the 

 alum card similarly placed proved intolerably hot. 



This result was corroborated by the following experiments : — 

 One bulb of a differential thermometer was covered with iodine, 

 and the other with alum powder. A red-hot spatula being 

 placed midway between both, the liquid column associated with 

 the alum-covered bulb was immediately forced down, and main- 

 tained in an inferior position. Again, two delicate mercurial 

 thermometers had their bulbs coated, the one with iodine, the 

 other with alum. On exposing them at the same distance to 

 the radiation from a gas-flame, the mercury of the alum-covered 

 thermometer rose nearly twice as high as that of its neighbour. 

 Two sheets of tin were coated, the one with alum, and the other 

 with iodine powder. The sheets were placed parallel to each 

 other, and about 10 inches asunder; at the back of each was 



* From the Philosophical Transactions for 1866, Part I. 



