_Answer._ Yes, very great. The difference in the _conducting_ power of wood and iron is shown if we place one end of a bar of each in the fire. The wood will be consumed without warming the bar more than a few inches from the fire, whereas the iron will soon become hot two or three feet from the fire. Owing to the difference in the conducting power of cotton and wool, we wear cotton clothing in summer and woolen in winter, because cotton allows the heat of the body to be conducted away from it, whereas woolen cloth prevents to a great degree this loss of heat. For the same reason, the venders of roasted chestnuts on our streets wrap them in a piece of blanket to keep them hot, that is, to keep the heat in; and in summer we wrap ice in the same way to keep it cold, that is, keep the warmth of the air out. The wool, being a very bad conductor of heat, simply prevents the heat from being transferred from the inside to the outside, and _vice versa_. It is for this reason that steam boilers, pipes and cylinders are nearly always covered with wood, and sometimes with felt. The difference in the _radiating_ power of various substances can be shown if we take a large thermometer and heat it up to the temperature of boiling water. If this thermometer is hung up in a room having the temperature of melting ice, it will lose heat in two ways,--first by heating the air which surrounds it, that is by _convection_, and also by _radiation_. In order to confine ourselves to the latter process, we will suppose that the chamber is a vacuum. If we first cover the bulb of the thermometer with a thin coating of polished silver, and then ascertain how much heat it radiates in a minute, and then coat it with lamp-black, and repeat the same experiment,--that is to say, allow the thermometer at the boiling-point to cool for one minute in a vacuum chamber at the freezing-point,--it will be found that the thermometer loses much more in a minute when coated with lamp-black than it did when coated with silver, showing that much more heat is radiated from a surface covered with lamp-black than from polished silver. Generally it may be stated that polished metals radiate much less heat than surfaces which are not polished.[7] For this reason, as well as for ornament, locomotive and other boilers and cylinders are usually covered with Russia iron or polished brass. [7] The account of the above experiment is copied from Balfour Stewart’s very excellent little book, “Lessons in Elementary Physics,” of which, and the same author’s “Elementary Treatise on Heat,” the writer has made frequent use. PART III. ON WORK, ENERGY, AND THE MECHANICAL EQUIVALENT OF HEAT. QUESTION 28. _For what purpose are all steam engines used?_