792 THE POPULAR SCIENCE MONTHLY. 



The results were not in all cases consistent with each other, but they 

 indicate that the property is dependent on the texture of the substance 

 rather than on the kind of material, or as concerns non-luminous heat 

 its color. 



The most recent experiments are those of Dr. Krieger, some results 

 of which are cited by Dr. Pettenkofer. He observed the rate of 

 cooling of a sheet-iron cylinder filled with hot water and covered by 

 turns with different cloths. Wrapping it with successive envelopes 

 of wool, buckskin, silk, cotton, and linen, and observing regularly the 

 diminution of temperature in a given time, he found the differences 

 insignificant, not exceeding one or two per cent. The color of the 

 materials did not cause the results to vary any more. It appears, 

 then, that in a dark heat the emissive power and the absorbing power, 

 which is correlative with it, vary but little between one kind of cloth 

 and another. The case is different when we have to do with luminous 

 heat, or the solar rays. With envelopes of linen, cotton, flannel, and 

 silk, M. Krieger observed that the absorption of solar heat increased 

 in the proportions indicated by the numbers 90, 100, 102, and 108. 

 The influence of color was much greater : with cotton goods of dif- 

 ferent hues he found the numbers to be white, 100 ; straw-color, 

 102 ; yellow, 140 ; bright green, 155 ; dark green, 168 ; Turkey red, 

 165 ; bright blue, 198 ; black, 208. These facts explain why in the 

 hot sun a black coat is warmer than a white one, while the difference 

 disappears in the shade. The influence of colors on the absorbing 

 powers of surfaces had already been made clear by the researches of 

 Leslie and Melloni. 



To form an estimate of the part which the conductibility proper 

 of the different materials plays in these phenomena, M. Krieger in- 

 quired how much the loss of caloric was diminished when the cylinder 

 was covered with double layers of the same cloths. The doubling of 

 the satin, cotton cloth, and fine linen diminished the loss only by from 

 three to six per cent, while doubling the envelopes of buckskin, flan- 

 nel, and woolen cloth diminished it by ten, twenty, and even thirty 

 per cent. It is clear from these experiments that the resistance offered 

 by cloths to the passage of heat depends much less on the conducti- 

 bility of the fibers that form their substance than on the thickness, the 

 volume, and the texture of the tissues. This can also be shown in 

 observing the cooling of a cylinder covered with wadding, which is 

 forty per cent more rapid when the wadding is strongly compresesd. 

 So a dressing-gown lined with wadding and a flannel waistcoat are 

 warmer when we first put them on than after they have been worn for 

 some time. The packing which the filaments undergo with use ren- 

 ders the cloth more permeable to heat. Although doubling the en- 

 velope has little influence when both layers are stretched tight over 

 the cylinder, it is not the same when a slight space is left between 

 them ; then the cooling is retarded thirty or thirty-five per cent by 



