In selecting materials for various purposes, account has to be taken of their conductivities, for in some cases it is desirable that the transfer of heat should take place slowly, and in others that it should take place quickly. It might be thought that the conductivity of a substance could be estimated by touch, but a little reflection will show that this cannot be the case. The flow of heat between two bodies depends upon the difference of temperature between them, and if there should be no difference of temperature between them at the moment of touch there will be no flow of heat, though both are bodies of greater or less conductivity. Let us take, for example of the uncertainty of estimation by touch, a well-known experiment. Suppose we have a basin of hot water and a basin of cold water, and place a hand in each for a few moments; suppose we withdraw the hands and plunge them into a basin of tepid water, we shall find that the tepid water feels cold to the hand that was in the hot water and warm to the hand that was in the cold water. Luckily, it has been found possible in the laboratory to refer substances to a common standard and to assign numerical values to them in order of their conductivities, so that substances can be compared and a selection made for any desired purpose. Pure silver has the highest conductivity; other useful materials take the following order: copper, zinc, lead, iron, steel, marble, glass, brick, slate, wood, fur, cotton, flannel, water, air. Fur and wool no doubt owe much of their warmth to the fact that they consist of fibres which enclose a good deal of air, but as a matter of fact the warmth of loosely woven woollen and knitted articles in general is often overrated; they are very warm as under garments or in calm weather, but in windy weather the air in them is rapidly changed and the cold seems to blow through them. If for any purpose we select a material from its place in a table of comparative conductivities, and use it without reference to the law of conduction of heat, we shall probably be disappointed with the result. We know that cotton burns easily; if we stretch a cotton handkerchief over the back of a gold watch and place a red-hot cinder from the fire on the handkerchief on the watch, the handkerchief will not be burnt.