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V. Sixth Memoir on Radiation and Absorption. — Influence of Colour and Mechanical 

 Condition on Radiant Heat. By Professor J. Ttndall, LL.jD., F.R.S., Member 

 of the Academies and Societies of Holland, Geneva, Gottingen, ZUrich, Halle, 

 Marburg, Breslau, Upsala, Cherbourg, la Societe Philomathique of Paris, Cam. Phil. 

 Soc. &c. ; Professor of Natural Philosophy in the Royal Institution and the Royal 

 School of Mines. 



Keceived December 21, 1865, — Kead January 18, 1866. 



Feanklin placed cloths of various colours upon snow and allowed the sun to shine upon 

 them. They absorbed the solar rays in dilFerent 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 qualifica- 

 tion. In my last memoir I briefly pointed out its probable defects. Did the emission 

 from luminous sources consist exclusively 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 Feanklin were the expression 

 of a law of nature. Tw© 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 tempera- 

 ture due to their position, it was found that the card bearing the alum became extremely 

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

 ential 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 maintained 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 



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