648 THE POPULAR SCIENCE MONTHLY. 



as compared with cotton or wool must be owing mnch less to its 

 own greater specific conducting power than to the smoothness 

 and inelasticity of its fibers. It has too slight a hold on the in- 

 cluded air. The more finely shredded it is the better it works ; 

 but our experiments have proved that it is not to be recommended 

 as a non-conductor. And yet asbestus is often spoken of as though 

 its excellence in this respect were unquestionable ; but, because 

 this wonderful mineral is very useful in many ways by reason of 

 its incombustibility, it does not follow that it has any magic vir- 

 tue in its other relations to heat. Asbestus paper intercepts heat 

 somewhat better than the loose fiber ; but a great many layers 

 must be put together, and then the virtue is by no means com- 

 mensurate with the cost. It is sometimes recommended as a 

 suitable article to put between floors to prevent the spreading of 

 a possible fire ; but those who propose it for this use seem to over- 

 look the fact that the efficiency of non-conductors is nearly pro- 

 portional to their thickness, and, though an inch might be of some 

 service, one fiftieth of an inch can do very little good. 



Fibrous matters and powders in the loose state are somewhat 

 troublesome to confine in the form of coverings, and hence they 

 are sometimes consolidated into sheets or blocks which can be 

 handled without breaking and applied easily. Hair-felt, which is 

 made in thick sheets from the hair which tanners scrape from 

 hides, is cheap and is very serviceable when the heat is not scorch- 

 ing. Paper pulp has been formed into very thick, hollow, half 

 cylinders to put around steam -pipes. Carbonate of magnesium 

 and fossil meal cohere when moistened and slightly compressed, 

 and they may be made into slabs with the addition of a very 

 small percentage of hair or asbestus to give toughness. Such a 

 paste may be plastered directly on steam pipes or boilers and 

 allowed to dry, the fiber serving to prevent cracking ; but the 

 greater compactness of light materials so consolidated renders 

 them less effective, especially when a heavy cementing substance 

 is added, like clay or plaster of Paris. 



Of non-conducting substances that are already in the solid 

 form, the light woods are often used advantageously. It should 

 be noticed that most of them conduct heat much better along the 

 grain than across it. Thus a cross-section of Liriodendron, or 

 yellow poplar, was found to transmit heat nearly twice as fast as 

 a board of the same thickness sawed lengthwise. Cork is expen- 

 sive and hard to get in large pieces ; but it is far preferable to 

 wood, as it is lighter and more elastic and does not absorb water. 

 Very porous and light bricks confine heat much better than those 

 that are hard burned, but they must be kept dry. 



The presence of moisture in a non-conductor greatly impairs 

 its usefulness, as every one knows who has attempted to hold a 



