CALORIC. 17 



mersing the bulb in boiling water; the freezing point, by immersing 

 it in melting ice. The distance between these two points is marked 

 in various modes; in Fahrenheit's scale, the freezing point is marked 

 32°, and the boiling point, 212° ; the intermediate divisions being ISO 

 degrees. In the centigrade thermometer, (Celsius',) the freezing 

 point is zero, and the boiling point 1 00°. In Reaumur's, the freezing 

 point is zero, and the boiling point 80°. As the ratio between these 

 three scales is that of 180, 100, and 80, or 9, 5, 4, it is easy to re- 

 duce one to the other. The alcoholic thermometer is used when 

 extremely low temperatures are to be measured. 



COMMUNICATION, OR TRANSFER OF HEAT. 



Heat may be communicated to bodies in different modes: — by 

 contact, as in the conduction of solids, and the circulation of liquids; 

 by radiation, and by reflection. 



Conduction of heat. — By this is meant the passage of heat from 

 one particle of a body to another. This conducting power is very 

 different in different bodies ; metals are the best conductors of heat, but 

 not all equally so; gold is the best conductor, lead the worst; glass 

 and porcelain are worse conductors than the metals. Liquids and 

 gases are nearly destitute of conducting power, as may be shown by 

 applying heat to the top of them; the mode of heating them is to 

 apply the heat at the bottom, when, instantly, there are two currents 

 set in motion, the hot particles rising towards the surface, and the 

 colder ones descending: this process is termed circidation. 



Radiation of heat. — That mode by which heat is given off to 

 some distance on all sides, by a heated body, is termed radiation; 

 and the heat, radiant lieat. Heat is emitted from a hot body in all 

 directions, and in straight lines or radii, just as rays of light are 

 emitted from a luminous body. These calorific rays pass freely 

 through the air, or a vacuum, without sensibly affecting its tempera- 

 ture. When they fall upon the surface of a solid body, they may 

 be disposed of in three different ways: — 1, they may be reflected; 

 2, they may be absorbed; or 3, they may pass through it, or be 

 transmitted. In the first and third cases, the temperature of the 

 body is unaffected ; in the second case, it is elevated. 



The radiating power of bodies varies very much ; it depends 

 chiefly upon the nature of their surfaces, those which are rough and 

 soiled throwing out more heat than those which are smooth and 

 polished ; lamp-black is the best radiator, polished metals the worst. 

 Colour alone has no influence upon it. 



Reflection of heat resembles reflection ,of light ; — familiarly shown 

 by holding a sheet of polished metal near the fire at an angle ; the 

 heat will be reflected in the corresponding angle ; it may be better 



2* 



