276 SCIENTIFIC AMUSEMENTS. 



be produced by the heat. The handles of metal teapots and 

 coffeepots are frequently made separate from the body of the 

 vessel, and united thereto with rings of ivory, or ebony, &c., between 

 the ends of the handle and its attachment to the pot, so as to 

 diminish the heating of the handle by the conduction from the 

 hot pot, the wood or ivory being comparatively an extremely bad 

 conductor. 



It is somewhat remarkable that those bodies which conduct 

 heat best (silver, copper, &c.,) are also those which conduct 

 electricity the best, and vice versd. 



Expt. 323. Different conducting power of Wood and Metal. 

 The difference in conductivity of heat subsisting between wood 

 and metal, especially copper, may be readily illustrated in a 

 simple way. Provide a cylindrical rod of wood some inches long 

 and f to 1 inch in diameter (part of a broomstick will do). 

 Slightly cut away the outer part of one half, and bend a piece of 

 stout sheet copper over the remaining wood at the cut end, so as 

 to cover the wood with copper, keeping the outside diameter about 

 the same as at first (fig. 129). Strain a piece of white writing 



paper over the prepared rod, 

 holding it so that one fold 

 of paper covers both the 

 wooden end and the other 

 Fig. 129. Conduction Wood and Metal metal-coated half, and then 

 compound Bar. pkce ^ rod and paper ^ 



the flame of a spirit lamp or Bunsen gas lamp, so that the flame 

 may impinge on the paper at about the centre. In a few seconds 

 that part of the paper in contact with the wood will be charred 

 and set on fire, whilst the other part in contact with the metal 

 will remain unaffected by the heat; the reason being that the 

 metal conducts heat away rapidly, and consequently cools the 

 paper and prevents its becoming heated to the charring point for 

 some time, whilst the wood is incapable of thus protecting the 

 paper, on account of its bad conducting power. 



Expt. 324. Effect of Wire Gauze in stopping the passage of 

 Flame. The conducting power of metals in cooling down con- 

 bustible substances, and so extinguishing flame by preventing the 

 temperature continuing high enough to propagate the chemical 

 action of burning, is utilised in the construction of various forms 

 of miners' safety lamps for the purpose of diminishing the liability 

 to dangerous explosions of fire damp (Expt. 208) in collieries and 

 underground workings. If a mixture of oxygen and hydrogen be 

 made in a bladder, in the mouth of which is fixed a piece of J inch 

 metal gas pipe some few inches long, on applying a light to the end 



