NON-CONDUCTORS OF HEAT. 649 



hot body with a damp cloth. Count Rumford, who long ago did 

 much valuable work in the experimental study of heat, concluded 

 that fluids have no conducting power at all, but transmit heat 

 solely by convection ; and, accordingly, water is still sometimes 

 spoken of as an exceedingly poor conductor. But later investi- 

 gators have disproved the correctness of that idea. Our own 

 trials show that, when convection is obviated, water transmits in 

 a given time six times as much heat as hair-felt of the same 

 thickness, and nearly eight times as much as still air. Others 

 have found that bisulphide of carbon and ether transmit heat 

 even better than water; but most liquid substances are slower 

 conductors. Thus it takes more than twice as long for a given 

 amount of heat to pass through cotton-seed oil or lard oil as 

 through water. 



As to the gases, some physicists seem to have proved that heat 

 passes through air more readily than through a vacuum, while 

 hydrogen has six times as much transmissive power, and carbonic 

 acid half as much as air ; but none of them used apparatus that 

 would give absolutely certain results. 



To show more clearly the retentive power of various sub- 

 stances, we subjoin the following table, in which the first column 

 of figures shows the net percentage of solid matter in a given 

 space. The second column of figures gives the number of English 

 units of heat transmitted in one hour through one square foot of 

 the covering one inch thick, the average difference of temperature 

 between the heater and the water in the calorimeter being 100 

 Fahr. By the English unit of heat is meant as much heat as will 

 raise the temperature of one pound of water 1 Fahr. Of course, 

 the smaller the number in the last column the better is the sub- 

 stance for keeping a body warm or cold. 



In some of the experiments the source of heat was steam at 

 310 Fahr. In the others a stream of water at about 176 Fahr. 

 was kept running through the heater. 



It is plain that in choosing non-conductors for practical serv- 

 ice we should take into account something more than their heat- 

 retaining power. They should be of materials that are abundant 

 and cheap ; clean and inodorous ; light and easy of application ; 

 not liable to become compacted by jarring, or to change by long 

 keeping ; not attractive to insects or mice ; not likely to scorch, 

 char, or ignite at the long-continued highest temperature to which 

 they may be exposed ; not liable to spontaneous combustion when 

 partly soaked with oil; not prone to attract moisture from the 

 air ; and not capable of exerting any chemical action on surfaces 

 with which they are placed in contact. There is no one thing 

 which combines all the desirable good qualities, but there is a 

 considerable range of substances which fulfill most of the require- 



TOL. XXXTIII. 44 



