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. [Illustration: Fig. 213.]