HEAT, 4) 
5. Conduction and Radiation.—Just as light is the vibratory motion of 
the particles of a luminous body, and reveals itself by affecting the nerves 
of the eye; so it is this vibratory motion of the particles of a hot body, 
when communicated to the nerves, which causes the sensation of heat. 
These vibrations pass in all directions, and are hence called radiant heat. 
(See Optics, p. 25.) Again, as sound is transmitted by solids as well as 
by the air, so heat is transmitted by solids. When the end of a poker is 
placed in the fire, a vibratory motion is imparted to the atoms of that 
end ; but this motion is communicated from atom to atom, till the other 
end also becomes heated. Metals shew the greatest facility in the passing 
of heat in this way; in other words, they are the best conductors of heat. 
The principle on which heat is transmitted through a fluid, as described at 
p. 39, and represented in the figure there, is called convection, because the 
particles of the body change their position, and, as it were, convey the 
heat ; in the conduction of metals, no particle changes its position, the 
motion is merely passed from one to the other. 
Having hitherto treated of the heating of a body, we will now consider 
the process of cooling. 'The motion going on among the atoms of a heated 
body is communicated to the ether, and the heat is said to radiate; thus 
the hot body expends energy, the motion of its own atoms gradually 
diminishes, and it is said to cool. Suppose, then, it were desirable to 
keep it from cooling, what could be done to prevent it? If the hot 
body were covered with another, the heat must first be conducted 
through this covering before it can be radiated. Now, different bodies 
have different powers of conduction, so that if a hot body be covered with 
a bad conductor, it will be kept hot for along time. This is the object of 
wearing clothes—not to warm one’s body, but to keep its heat from being 
radiated. When a piece of red-hot metal is exposed to the air, the 
heat radiates from the outside, and the outer coating of cooled metal 
becomes a conductor to the heat in the interior. When the body 
cooling is a bad conductor, which is quite a different thing from the 
radiation of heat from the outside, the internal heat is preserved for a long 
time. The lava that runs as a red-hot liquid from volcanoes, and spreads 
out in great sheets, after cooling on the surface, so that people may walk 
over it, retains its heat under this crust for years, because it is a bad con- 
ductor. 
6. Evaporation and Dew.—One of the most interesting phenomena 
connected with heat is Drew, the nature of which will be perfectly 
intelligible after the above explanations. When treating of the heating 
of liquids, we saw that, with a degree of heat much less than what would 
raise a liquid to the boiling-point, vapour is formed at the surface of the 
liquid. The formation of vapour in this way is called evaporation. The 
heat of the sun is continually causing evaporation from all bodies of 
