OCR Text |
Show Kon-ronductors of Heat. Ground ork and some other barks, and the sawdust of the soft woods, aa well as the charcoal made of these substances, sub-stances, are very good retainers of heat. Lampblack alc;o works well. When the I thing to bo kpt hot is at a very high j temperature, some light, incombustible 1 powders are very suitable. Among the best of these aro fossil meal aud the cal- i cined magnesia and magnesium carbon- I ato of the ilrnr(Hita. E'nnqil miil pnn. i gists of the silicious skelotons of microscopic micro-scopic vegetables, culloil diatoms, exceedingly ex-ceedingly various in shape and size, the very largest of them hardly reai'liing the k'Utfth of the hundredth of an inch. It in found abundantly in some peat meadows mead-ows and in the bottoms of ponds. Both fossil meal and magnesium carbonate) have been largely used in covering steam pipes. Obviously, when tho same light substance sub-stance is tried in both the first and second sec-ond apparatus above mentioned, and the results differ, it must bo owing to the inability in-ability of the substance to hold the included in-cluded air still in the first arrangement. So powdered plumbago or black lead, J which is very slippery, shows nearly ! twice as much transmissive power in one ! case as in the other. Loosened asbestos ! fiber also lets through about twice as j much heat in the vertical arrangement na iu tho horizontal. Yet this fiber may be split up exceedingly fine, but the great 1 difference in its behavior as compared with cotton or wool must lie owing much I less to its own greater sjiecific conducting conduct-ing power than to the smoothness and inelasticity of its fibers. Professor John I M. Ordway in Popular Science Monthly, j |