Fig. 211.
297. Conduction.—In this mode of diffusion the heat goes through or among the particles of substances. For example, if one end of a bar of iron be held in the fire, it travels through or among the particles to the other end. The gradual progress of the heat may be seen by the following simple experiment: Take a rod of iron and attach to it, as seen in Fig. 211, some little balls of wood by means of wax. By heating one end with a lamp the balls will drop one after another as the heat passing along melts the wax which holds them.
Fig. 212.
298. Conductors and Non-Conductors.—Heat is conducted more rapidly through some substances than through others. There is great variety in this respect. There is considerable among those which are reckoned as good conductors, as is shown by the experiment represented in Fig. 212. Here are cones of the same size of seven different substances—copper, iron, zinc, tin, lead, marble, and brick—all tipped with a little wax, and placed on a stove. The wax will melt on the copper cone first, showing that this is the best conductor of them all; and on the brick one last, showing that this is the poorest conductor. The conducting powers of the rest are according to the order in which I have mentioned them.
Those substances which allow heat to pass through them very slowly are called non-conductors. The term, though convenient, is not a strictly correct one, for there are no substances which do not conduct heat in some degree. Wood is one of these poor conductors, and hence wooden handles are put upon various instruments and vessels that are used about fires, as the soldering irons of the tinman, the metallic tea-pot, etc. As cloth is a non-conductor, the holder is used in taking off the tea-kettle and in using the flat-iron. Glass is so poor a conductor that if you hold a rod or tube of it across the flame of a spirit lamp or gas burner, and heat it even to redness, you can place your fingers very near to the heated portion with impunity. I had occasion to-day to bend a small glass tube in this way, and I observed some water in it quite near to the heated part which remained undisturbed through the process. It is the non-conducting quality of glass that makes it so liable to break, when it is thick, if it be exposed to any sudden change of temperature. For example, if hot water be poured into a thick glass vessel, the inner surface is quickly expanded; but the outer surface not expanding with it, because the heat is not readily conducted through, this irregularity in expansion causes a fracture. It is for this reason that the flasks, retorts, etc., used by the chemist are made very thin, especially where the heat is to be applied.
Fig. 213.
299. Davy's Safety-Lamp.—One of the most beautiful applications of the conduction of heat we have in the Safety-Lamp of Sir Humphrey Davy, an invention which has been the means of saving the lives of multitudes of miners. It is represented in Fig. 213. With this lamp one can go into the midst of the most explosive gases with impunity. Now all that prevents the flame within from setting on fire the gases without is a covering of wire-gauze. This, being a good conductor, conducts off the heat of the flame within so rapidly that it can not go through the openings as flame, and so does not set fire to the gas without. The fact upon which the construction of this lamp was based was discovered by trying many experiments. Among them were the following: A piece of wire-gauze was held over a candle so that its flame struck against it. The smoke issued above, but no flame. Then a stream of gas was allowed to pass through the gauze, as seen in Fig. 214, and was set on fire above. It burned without inflaming the gas below.[4]
