88 



EXPERIMENTAL GENERAL SCIENCE 



heat may come from the organisms themselves Even plants 

 generate a small amount of heat (182). Respiration, as we 

 have seen, is really a slow burning which occurs in both animals 

 and plants. The lower animals usually have a temperature 

 not very different from their surroundings, but birds and mam- 

 mals have a temperature higher than that which ordinarily 

 prevails, and this temperature is maintained nearly uniform 

 whether the surroundings be hot or cold. In health, our own 

 bodies have a temperature of about 98.6F. ; and if anything 

 occurs to prevent the loss of the excess heat produced, we soon 

 have a fever. The sense by which we judge the temperature of 

 an object is very easily deceived. A piece of iron will feel 

 colder than a piece of wood on a cold morning, but the ther- 

 mometer proves it to be of the same temperature. On a hot 

 day iron will feel hotter than wood. The explanation of this is 

 that when we are losing heat rapidly we feel chilly, and when 

 we are gaining heat rapidly we feel warm. Metal being a 

 good conductor of heat, therefore, is apparently, but not 

 really, colder or hotter than wood (45). 



84. Uses of Radiation and Conduction. Many familiar 

 operations make use of the principles of radiation and con- 

 duction. In the ice cream freezer 

 the material to be frozen is put 

 into a metal vessel surrounded by 

 salt and ice. The metal, being a good 

 conductor, rapidly transfers the heat 

 from the cream to the melting ice. 

 The outside of the freezer is usually 

 made of wood, a poor conductor, 

 which prevents the ice from being 

 melted by the heat from outside. In 

 FIG. 30. A fireiess cooker, the fireless cooker, the heated food is 

 (Ahrens,Harie V and Burns.) surrou nded by a layer of hay, cork 

 dust, excelsior, or other material in which are many small 



