Page:Popular Science Monthly Volume 10.djvu/686

666 made by Dr. Krieger on wool, wash-leather, silk, cotton, linen, and India-rubber, have not shown any important difference. Krieger covered cylinders made of tin and filled with warm water with different and differently-arranged materials, and noted the decrease of temperature 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 different materials of the same color, but become great indeed when the colors are different. For white textures the following proportions are found:

With shirtings of different colors the proportions were:

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 considerably 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 evaporation), 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-conducting 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,