Holders.
Ice kept in flannel.
It is for this reason that wooden handles are put upon some iron tools that are used in operations about the fire. The tool which the tinman uses in soldering has a wooden handle. If it had not, his hand would be burned by the heat going up to it by the iron handle; but very little of it goes into the wooden handle and spreads there, because wood is so poor a conductor of heat. We do not need wooden handles for tongs and pokers, because we do not have to keep them in the fire so long as the tinman does his soldering-iron. The handle of a metallic tea-pot is, you know, made of wood; for, if it was metallic, the heat from the tea would spread through it, and make it so hot that it could not be held in the hand. The holder which is used in ironing is of service, because it is so poor a conductor of heat. The heat does not readily go through it to the hand; so, also, we sometimes use paper to take up things that are hot, because the paper, being a poor conductor, does not let much of the heat pass through it to the hand. You have seen people wrap up ice in flannel to keep it from melting. The flannel here does for the ice what the woolen or paper holder does for the hand—it prevents the heat in the air around from getting to the ice.
Experiment on a stove.
Here is represented an experiment which shows how heat spreads through different things with different degrees of rapidity. Some pieces of different things of the same size and shape are put on top of a stove. They are pieces of iron zinc, copper, lead, marble, and brick. On the top of each is put a little bit of wax. The wax on the copper melts first, because this is a better conductor of heat than any of the others. Next is the iron; next, the zinc; next, the lead; next, the marble; and last of all, the brick.
Air a poor conductor of heat.
In air that is kept still heat spreads very slowly; but heat, when it can, always sets air in motion. I have told you, in Chapter XIII., how heated air rises and cold air takes its place. This is going on all the time about a stove. As fast as the air is heated, it goes up by the stove and the pipe, and cold air keeps coming to the stove to be heated. In this way all the air in the room is, after a little while, warmed. Now, if the air could all be kept still instead of being kept in motion in this way, it would take a long time for the heat to be spread from the stove through it, for air, like wood, is a poor conductor of heat.
Double windows.