THERMOMETERS. These are not the only measurers of heat. We have the pyrometers, used for ascertaining the temperature of extremely hot bodies, and the thermo-electric pile, an apparatus which constitutes the most delicate test for heat which has been devised. It will detect heat in the body of a fly walking near it. [Illustration: SHOWING DISTILLATION. _Ex._—Place a small amount of water, colored with ink, in a flask, and apply heat. The water will be vaporized, and in passing through the tube, which is surrounded by another tube containing cold water, it is condensed as a colorless liquid.] Thermometers are of three kinds, as to the materials used. They are air, alcohol, and mercurial. In each case the contraction and expansion of these respective substances are made to register variations of heat and cold. They are of three kinds, as to their system of grading—Réaumur’s, the Centigrade, and Fahrenheit’s. The first two make zero the freezing point; the last makes 32°. The boiling point of Réaumur’s is 80°, the Centigrade 100°, and Fahrenheit’s 212°. Once more changing the basis of classification, we find thermometers divided into three classes, with reference to the purposes they serve. The ordinary thermometer records the degree of heat or cold at the moment of observation. The differential thermometers can be made of two ordinary thermometers, by wrapping a piece of cloth around the bulb of one; these would show at any given moment whether it was growing warmer or colder. If it is growing warm, the column of mercury in the thermometer with the covered bulb will stand lower than the other, as the cloth prevents the heat reaching the quicksilver as readily as in the other. If it is higher than in the other, the weather is growing colder, as the cover prevents the heat from going off as rapidly as from the other. The third class, the registering thermometer, is so called because it marks the extremes of temperature. Without going into detail, it is perhaps sufficient to say that a minute bar of steel is placed on top of the column of mercury, and remains at any point to which it is pushed, thus recording the greatest degree of heat during any given interval of time. Somewhat similar in arrangement is the alcohol thermometer, marking the greatest degree of cold. It will, of course, be understood that almost all apparatus is greatly varied to serve special purposes. The limits of our article will preclude further discussion of fire in relation to light, although the subject of both physical and physiological topics is full of fascination and value. _End of Required Reading for March._