274 SCIENTIFIC AMUSEMENTS. 



of flannel or loosely wrapped newspaper, which prevents the access 

 of warm air in this case, just as it would keep the human body 

 from chilling in a cold atmosphere. 



" Norwegian stoves," for keeping food hot a long time, are simply 

 boxes lined with a thick coating of felt or similar porous material, 

 which will thus prevent chilling. Ice chests are similarly coated 

 outside with a double jacket, either containing air only, or better 

 still, a layer of felt, dry sawdust, or similar material. Similarly, 

 fireproof safes are protected by a corresponding jacket filled with 

 sand, asbestos, mineral wool, or analogous material, capable of pre- 

 venting the easy passage inwards of heat, whilst not being itself 

 susceptible of charring. 



Expt. 321. Conduction of Heat. The property possessed by an 

 iron poker thrust into the fire of becoming more or less heated at 

 the handle end (even though this be screened from the radiant 

 heat of the fire) by the conduction of heat along its length, is shared 

 to a greater or less degree by all solids, whilst liquids and gases 

 possess it only to an extremely minute extent. If a copper rod 

 and another one of iron, both of the same dimensions (say 2 feet 

 long), be placed with one end of each in the fire, it will be found 

 that the far end of the copper rod will soon become too hot to grasp 

 comfortably with the hand, whilst the iron rod will be much less 

 affected, showing that the copper conducts heat better than 

 iron. Fig. 127 represents another mode of illustrating this. Two 



similar bars of 

 IRON different metals 



(such as copper 

 and iron) are 

 fastened to- 

 gether, end to 

 end, and a num- 



Fig. 127. Conduction (solids). . ber of l{ 8^ 



wooden balls 



cemented by means of melted beeswax to the under portion of 

 the compound bar at equal intervals (say 1 inch apart). The 

 central part where the two ends meet is then heated by means 

 of a lamp ; heat is conducted along each metal, causing the wax 

 to melt and the attached balls to drop off; the better conduct- 

 ing bar allows the heat to pass more rapidly and to a greater 

 distance than the other ; so that in the case of copper and iron 

 the balls attached to the copper rod will drop sooner, and more 

 of them will be ultimately detached than will be the case with 

 the iron rod. 



Expt. 322. Another Illustration. Fig. 128 illustrates another 



