Let us take, for instance, a crystal of tourmaline and raise its temperature, and we shall find one end positively, and the other negatively, electrified. Again, let us take the same crystal, and suddenly cool it, and we shall find an electrification of the opposite kind to the former, so that the end of the axis, which was then positive, will now be negative. Now, this separation of the electricities denotes energy; and we have, therefore, in such crystals a case where the energy of heat has been changed into that of electrical separation. In other words, a certain amount of heat has passed out of existence _as heat_, while in its place a certain amount of electrical separation has been obtained. 161. Let us next see under what circumstances heat is changed into _electricity in motion_. This transmutation takes place in thermo-electricity. Suppose, for instance, that we have a bar of copper or antimony, say copper, soldered to a bar of bismuth, as in Fig. 12. Let us now heat one of the junctions, while the other remains cool. It will be found that a current of positive electricity circulates round the bar, in the direction of the arrow-head, going from the bismuth to the copper across the heated junction, the existence of which may be detected by means of a compass needle, as we see in the figure. [Illustration: Fig. 12.] Here, then, we have a case in which heat energy goes out of existence, and is converted into that of an electric current, and we may even arrange matters so as to make, on this principle, an instrument which shall be an extremely delicate test of the existence of heat. By having a number of junctions of bismuth and antimony, as in Fig. 13, and heating the upper set, while the lower remain cool, we get a strong current going from the bismuth to the antimony across the heated junctions, and we may pass the current so produced round the wire of a galvanometer, and thus, by increasing the number of our junctions, and also by using a very delicate galvanometer, we may get a very perceptible effect for the smallest heating of the upper junctions. This arrangement is called the _thermopile_, and, in conjunction with the reflecting galvanometer, it affords the most delicate means known for detecting small quantities of heat. [Illustration: Fig. 13.] 162. The last transmutation on our list with respect to absorbed heat is that in which this species of energy is transformed into _radiant light and heat_. This takes place whenever a hot body cools in an open space--the sun, for instance, parts with a large quantity of his heat in this way; and it is due, in part at least, to this process that a hot body cools in air, and wholly to it that such a body cools in vacuo. It is, moreover, due to the penetration of our eye by radiant energy that we are able to see hot bodies, and thus the very fact that we see them implies that they are parting with their heat.