in water, was found to have about four times the transmitting power of loose cotton. Were the transmission due to the substance of the fibers themselves, it would be increased by packing more in the same space. But, in fact, it is found that it is somewhat diminished by moderate crowding.
It would appear, then, that the efficiency of light non-conductors must be owing mostly to the imprisoned air which really occupies all but a small fraction of the space; and the stiller the air is held, the better is the effect.
Of course, the amount of friction which fibers can oppose to the motion of the entrapped fluid depends on their minute structure and arrangement. Thus in cotton they are long, fiat, twisted, irregular in breadth, and variously bent. And as to fineness, it was found by counting and weighing some Sea Island cotton fibers averaging about an inch and a half in length, that it would take seventeen thousand to weigh a grain. Wool is scaly and very crinkly. Down is made up of flat threads with innumerable short, loose branches. The heads of the common cat-tail (Typlia latifolia), which make a good non-conductor, consist of brown seeds, each having a stalk with very spreading branches. The seeds, with their appendages, are so very fine that eight hundred of them weigh only one grain. They may well float, as each one, for its weight, presents a very extensive surface to the air; and, for the same reason, in mass they serve to keep the air stagnant.
Ground cork and some other barks, and the sawdust of the soft woods, as well as the charcoal made of these substances, are very good retainers of heat. Lampblack also works well. When the thing to be kept hot is at a very high temperature, some light, incombustible powders are very suitable. Among the best of these are fossil meal and the calcined magnesia and magnesium carbonate of the druggists. Fossil meal consists of the silicious skeletons of microscopic vegetables, called diatoms, exceedingly various in shape and size, the very largest of them hardly reaching the length of the hundredth of an inch. It is found abundantly in some peat meadows and in the bottoms of ponds. Both fossil meal and magnesium carbonate have been largely used in covering steam-pipes.
Obviously, when the same light substance is tried in both the first and second apparatus above mentioned, and the results differ, it must be owing to the inability of the substance to hold the included air still in the first arrangement. So powdered plumbago, or black lead, which is very slippery, shows nearly twice as much transmissive power in one case as in the other. Loosened asbestus fiber also lets through about twice as much heat in the vertical arrangement as in the horizontal. Yet this fiber may be split up exceedingly fine; but the great difference in its behavior
