POPULAR FALLACIES. 



89 



body is defended from the effects of the hotter part of the surrounding atmo- 

 sphere. A carpet, being a bad conductor of heat, fails to transmit heat to the 

 foot, and therefore, though at a higher temperature than the body, creates no 

 sensation of warmth. The tiles and marble, being better conductors of heat, 

 and at a higher temperature than the body, transmit heat readily, and metallic 

 objects still more so : these, therefore, feel hot. On passing into a cold room, 

 the very contrary effects ensue. Here all the objects have a temperature be- 

 low that of the body ; the carpet and other bad conductors, not being capable 

 of receiving heat when touched, produce no sensation of cold ; wood, being a 

 better conductor, feels cooler ; marble, being a better conductor, gives a still 

 stronger sensation of cold ; and metal, the best of all conductors, produces that 

 sensation in a still greater degree. 



In cold temperatures, the particles of water which carry off the heat from 

 the body are far more numerous than those of air, and therefore carry the heat 

 off more rapidly ; and, besides, they are constantly changing their position ; 

 the particles warmed by the body immediately ascend by their levity, and cold 

 particles come into contact with the skin. Thus water, although a bad conductor 

 of heat, has the same effect as a good conductor, by the effect of its currents. 



Sheets feel colder than the blankets, because they are better conductors of 

 heat, and carry off the heat more rapidly from the body ; but when, by the con- 

 tinuance of the body between them, they acquire the same temperature, they 

 will then feel even warmer than the blanket itself. Hence it may be under- 

 stood why flannel, worn next the skin, forms a warm clothing in cold climates, 

 and a cool covering in hot climates. 



To explain the apparent contradiction implied in the fact that the use of a 

 fan produces a sensation of coldness, even though the air which it agitates is 

 not in any degree altered in temperature, it is necessary to consider that the 

 air which surrounds us is generally at a lower temperature than that of the 

 body. If the air be calm and still, the particles which are in immediate con- 

 tact with the skin acquire the temperature of the skin itself, and, having a sort 

 of molecular attraction, they adhere to the skin in the same manner as particles 

 of air are found to adhere to the surface of glass in philosophical experiments. 

 Thus sticking to the skin, they form a sort of warm covering for it, and speed- 

 ily acquire its temperature. The fan, however, by the agitation which it pro- 

 duces, continually expels the particles thus in contact with the skin, and brings 

 new particles into that situation. Each particle of air, as it strikes the skin, 

 takes heat from it by contact, and, being driven off, carries that heat with it, 

 thus producing a constant sensation of refreshing coolness. 



Now from this reasoning it would follow that, if we were placed in a room 

 in which the atmosphere has a higher temperature 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. 



It may be objected to some of the preceding reasonings, that glass and por- 

 celain, 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 sur- i 

 face of the glass, or perhaps the particles to some very small depth within it, j 

 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 J 



