NON-CONDUCTORS OF HEAT. 647 



in water, was found to have about four times the transmitting 

 power of loose cotton. Were the transmission due to the sub- 

 stance of the fibers themselves, it would be increased by packing 

 more in the same space. But, in fact, it is found that it is some- 

 what diminished by moderate crowding. 



It would appear, then, that the efficiency of light non-conduct- 

 ors must be owing mostly to the imprisoned air which really oc- 

 cupies 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 struct- 

 ure 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 reach- 

 ing the length of the hundredth of an inch. It is found abun- 

 dantly 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 dif- 

 fer, it must be owing to the inability of the substance to hold the 

 included air still in the first arrangement. So powdered plum- 

 bago, 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 



