666 THE POPULAR SCIENCE MONTHLY. 



made by Dr. Krieger on wool, wash-leather, silk, cotton, linen, and 

 India-rubber, have not shown any important difference. Krieger cov- 

 ered cylinders made of tin and filled with warm water with different 

 and differently-arranged materials, and noted the decrease of tempera- 

 ture in stated periods. He used layers of two different materials, but 

 it made no great difference what the outer layer was. Still, I will 

 mention that silk and cotton allowed more heat to radiate than 

 wool. The color also of the material has been shown to have no 

 great influence on the radiation of heat, which remains the same, 

 whether we have a black or a white garment on. 



But it is quite another case when we receive luminous heat, rays of 

 heat proceeding from luminous bodies, such as the sun, or some flame ; 

 then differences result, which certainly are not very great with differ- 

 ent materials of the same color, but become great indeed when the 

 colors are different. For white textures the following proportions 

 are found : 



When cotton received 100 



Linen received 98 



Flannel " 102 



Silk " 108 



With shirtings of different colors the proportions were : 



White 100 



Pale straw-color 102 



Dark yellow 140 



Light green 155 



Dark green 168 



Turkish red 165 



Light blue 198 



Black 208 



Of course, you all know by experience that, when dressed in black, 

 you feel much hotter in the sun than when dressed in white. It is 

 remarkable that, pale straw excepted, each color heightens consider- 

 ably the absorption of luminous heat-rays, and that blue does so 

 nearly as much as black. But, as soon as we are in the shade, the 

 differences nearly vanish. 



If we continue to consider our loss of heat by radiation through 

 and from our clothing (omitting for the present conduction and evap- 

 oration), we come at once to the practical question, how much this 

 loss is retarded by interposing several strata of material between our 

 surface and the air, or in fact to the question about the heat-conduct- 

 ing power of materials and textures. Very few experiments have 

 been made in this respect. We know, with respect to this point, the 

 properties of metals, of minerals, of chemical compounds, but not of 

 wool, linen, or leather. This shows, by-the-by, how little hygiene 

 has been treated until now in an exact and scientific way. We talk 

 in a general way about the use of garments as bad conductors of heat, 



