CONDUCTION OF HEAT. 275 



way of showing the same thing. A box is provided (a cigar box 

 made water tight with putty will do), in the side of which are bored 

 a number of holes in the same hori- 

 zontal line. In each hole is fixed a 

 cork, through which passes a rod of 

 metal, &c., a few inches in length. 

 One rod may be of copper, another of 

 brass, another of iron, another of glass, 

 and so on, all being as nearly as pos- 

 sible of the same dimensions; each 

 rod is so arranged that equal lengths 

 always project outwards from the Fi S- l * s - Conduction of 

 , J 1-1 , J -i -j Heat (solids), 



box, whilst an inch or more is inside 



it. The external portions of the rods are coated over with 

 melted beeswax by a brush, and small balls of wood cemented on 

 to the far ends by the same means ; after cooling and standing to 

 harden the wax, boiling water is poured into the box so as to cover 

 the ends of the rods inside ; this heats the rods and the heat is 

 conducted along them outside, melting the wax, and finally causing 

 the ball to drop off at the far end, if the rod is of sufficiently good 

 conducting material. It will then be seen that copper conducts 

 much better than iron, iron better than zinc or tin, and glass or 

 slate very little ; so that with these substances the heat travels far 

 enough to melt only a very small quantity of wax. 



Owing to the low conducting power of glass, slate, &c., these 

 substances may be rendered extremely hot by means of a flame at 

 one spot, whilst only a very short distance away the material 

 remains cool enough to hold in the hand without inconvenience. 

 Thus, by the aid of a blow pipe a candle flame may be directed on 

 the end of a piece of glass rod held horizontally so as to heat it 

 red hot and even to melt it ; whilst less than an inch away from 

 the softened glass the rod may be readily held in the fingers. 



On the other hand, iron, and especially copper and brass, conduct 

 heat so readily that tools made of these metals require to be fitted 

 with wooden handles if the tools are intended for use in a heated 

 condition," so as to prevent the hand being burnt; e.g., the 

 soldering tool referred to in Expt. 47, or an ordinary pair of ladies' 

 hair curling tongs. For a similar reason the flat irons used by 

 laundresses for smoothing starched collars, &c., and kettles of boil- 

 ing water, are grasped by means of a folded piece of cloth or 

 similar " holder " to prevent the heat passing to the hand so rapidly 

 as to scorch it ; an extremely hot plate may be handled safely at 

 table if the hand be protected by a glove or by catching hold of 

 the plate with a napkin ; whereas, if directly grasped, pain would 



