646 THE POPULAR SCIENCE MONTHLY. 



Therefore the heater is suspended with its used face downward 

 and exactly horizontal. The calorimeter, with its face also hori- 

 zontal, is placed at any chosen distance "below the heater, and is 

 furnished with a curb of well-varnished pasteboard extending up 

 a little higher than the face of the heater. This curb is of a 

 somewhat larger diameter than the hot box, so that there is a 

 free space all around, and very little heat can be conducted by it. 



Thirdly, in making practical tests of coverings for steam- 

 pipes the non-conductor is put entirely around the pipe and the 

 calorimeter is made in two parts with concave sides to fit the cov- 

 ering. Of course, in all cases the whole apparatus is surrounded 

 by cotton-wool or woolen blankets to prevent the disturbing in- 

 fluence of the surrounding air. 



With the first arrangement, if the space between the calorim- 

 eter and the heater is filled with air only, which is confined by 

 a curb of paper, but is free to circulate within the inclosure, the 

 heat passes over rapidly, especially when the heater is at a very 

 high temperature, while in the second apparatus the transmission 

 is slow. In the former case, convection has full scope; in the 

 latter, the air is stagnant and the heat passes downward by con- 

 duction and radiation. Therefore, still air has very little trans- 

 mitting power, and confined air which is free to move around 

 within the inclosure conveys heat readily. 



Yet it is a not uncommon belief that, as air is a poor conductor 

 of heat, a mere inclosed air-space around a hot or a very cold body 

 suffices to prevent change of temperature. It is said by some 

 that an ice-pitcher or a refrigerator needs only a double wall and 

 no filling between. And we occasionally meet with loose state- 

 ments like the following : " Confined air has long been regarded 

 by scientific and practical men as one of the best non-conductors 

 of heat." But it should be remembered that imprisonment is not 

 always close confinement. The air must be fettered so that it can 

 not stir. 



Now, if we fill the space in either the first or the second appa- 

 ratus with cotton or fine wool, we shall find the transmission even 

 less than with still air. And yet the fibrous matter may actually 

 occupy only a hundredth part of the space which it apparently 

 fills, and the fibers can touch the heated surface and each other 

 only in a few points. Therefore the specific conducting power of 

 wool or cotton can have very little to do with their capability of 

 keeping back heat. We know not precisely what the conducting 

 power of the solid matter of cotton may be, for we can not com- 

 press the fibers far enough to destroy their elasticity and expel all 

 the included air. But the woods are very similar in substance, 

 and some of them are two thirds as dense as fully compacted cot- 

 ton would be. One of the dry, hard woods, heavy enough to sink 



