LXXII.—HEAT:—CONDUCTION AND RADIATION.
In a former lesson you read a description of the thermometer, a useful instrument which enables us to estimate the temperature, or sensible heat, of substances.
All bodies, even the coldest, contain heat; and they have also a tendency to part with their heat to colder substances around them, until all have the same temperature. Thus, when you lay your hand on a block of iron or marble, heat leaves your hand to enter the less warm material and raise its temperature; and it is this abstraction of heat that produces in you the sensation of cold.
There is, then, a constant communication or transmission of heat from one body to another. This communication is effected chiefly in two ways—by conduction and by radiation. In conduction, the bodies are in contact; in radiation, they are at some distance apart.
If you push one end of a cold poker into the fire, that end will soon become warm, and the heat will be propagated from particle to particle through the poker, until the end most distant from the fire becomes too hot to be touched without injury. This mode of transmission is called conduction. Different substances possess this power in very different degrees. Thus, if instead of a poker you thrust into the fire a bar of wood of equal length and thickness, you will find that, even when the inserted end is in flames, the other remains comparatively cold, and may be handled with impunity. Hence we say that iron is a good conductor, and wood a bad conductor of heat.
The conducting power of bodies depends in a great measure on the closeness of their particles—dense, solid substances being much better conductors than those which are light and porous. The metals are the best conductors, but they differ very much among themselves. The best is silver; the others stand in this respect in the following order—copper, gold, brass, tin, iron, steel, lead.
You will now understand why metals feel cold to the touch: it is because, being good conductors, they carry the heat rapidly away from that part of our body with which they are in contact.
Among the bad conductors of heat are fur, wool, cotton, silk, and linen; straw, paper, feathers, wood, earth, snow, water, and air; and loose bodies, such as sawdust and shavings, which contain a large amount of air in the spaces between their particles.