FIRE-EATERS AND MOUNTEBANKS. 



placed in a room in which the atmosphere has a higher tempera- 

 than 96, the use of a fan would have exactly opposite effects, and, 

 instead of cooling, would aggravate the effects of heat ; and such 

 would, in fact, take place. A succession of hot particles would, 

 therefore, be driven against the skin, while the particles which 

 would be cooled by the skin itself would be constantly removed. 



12. It may be objected to some of the preceding reasonings, 

 that glass and porcelain, though among the worst conductors of 

 heat, generally feel cold. This, however, is easily explained. 

 When the surface of glass is first touched, in consequence of its 

 density and extreme smoothness, a great number of particles 

 come into contact with the skin ; each of these particles, having a 

 tendency to an equilibrium of temperature, takes heat from the 

 skin, until they acquire the same temperature as the body which 

 is in contact with them. When the surface of the glass, or perhaps 

 the particles to some very small depth within it, have acquired 

 the temperature of the skin, then the glass will cease to feel cold, 

 because its bad conducting power does not enable it to attract 

 more heat from the body. In fact, the glass will only feel cold 

 to the touch for a short space of time after it is first touched. 

 The same observation will apply to porcelain and other bodies 

 which are bad conductors, and yet which are dense and smooth. 

 On the other hand, a mass of metal, when touched, will continue 

 to be felt cold for any length of time, and the hand will be inca- 

 pable of warming it, as was the case with the glass. 



A silver or metallic tea-pot is never constructed with a handle 

 of the same metal, while a porcelain teapot always has a porcelain 

 handle. The reason of this is, that metal being a good conductor 

 of heat, the handle of the silver or other metallic teapot would 

 speedily acquire the same temperature as the water which the 

 vessel contains, and it would be impossible to apply the hand to 

 it without pain. On the other hand, it is usual to place a 

 wooden or ivory handle on a metal teapot. These substances 

 being bad conductors of heat, the handle will be slow to take 

 the temperature of the metal, and even if it does take it, will 

 not produce the same sensation of heat in the hand. A handle, 

 apparently silver, is sometimes put on a silver teapot, but, it 

 examined, it will be found that the covering only is silver ; and 

 that at the points where the handle joins the vessel, there is a 

 small interruption between the metallic covering and the metal 

 of the teapot itself, which space is sufficient to interrupt the 

 communication of heat to the silver which covers the handle. 

 In a porcelain teapot, the heat is slowly transmitted from the 

 vessel to its handle ; and even when it is transmitted, the handle, 

 being a bad conductor, may be touched without inconvenience. 



95 



