Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

water may be spilt. We may also make use of an egg-shell as a little vessel in which to heat the water, by resting it on a wire ring over the flame of the spirit lamp.

      Fig. 78.—Experiment on the regelation of ice.

      Fig. 78 shows the arrangement of a very remarkable experiment, but little known, on the refreezing of ice. A block of ice is placed on the edge of two iron chairs, and is encircled by a piece of wire, to which is suspended the weight of say five pounds. The wire penetrates slowly, and in about an hour’s time has passed completely through the lump of ice, and the weight, with the piece of wire, falls to the ground. What happens then to the block of ice?—You imagine, doubtless, that it is cut in two. No such thing; it is intact, and in a single lump as it was previous to the experiment. In proportion as the wire was sunk through the mass, the slit has been closed again by refreezing. Ice or snow during the winter may serve for a number of experiments relating to heat. If we wish to demonstrate the influence of colours on radiation, we take two pieces of cloth of the same size—one white, and the other black—and place them both on the snow, if possible, when there is a gleam of sunlight. In a short time it will be found that the snow underneath the black cloth has melted to a much greater extent than that beneath the white cloth, because black absorbs heat more than white, which, on the contrary, has a tendency to reflect it. We perceive very plainly the difference in temperature by touching the two cloths. The white cloth feels cold in comparison with the black cloth.

      It is hardly necessary to point out experiments on the expansion of bodies. They can be performed in a number of different ways; by placing water in a narrow-necked bottle, and warming it over the fire, we can ascertain the expansion of liquids under the influence of heat. We may in this way construct a complete thermometer.

      We may now consider the Sources of Heat, or causes of its development, which are various, and in many cases apparent. The first great source is the Sun, and it has been calculated that the heat received by the earth in one year is sufficient to melt an envelope of ice surrounding it one hundred and five feet thick. Of course the heat at the surface of the sun is enormously greater than this, about one-half being absorbed in the atmosphere before it reaches us at all. In fact, it is impossible to give you an idea of the enormous heat given out by the sun to the earth (which is a very small fraction indeed of the whole), stars, and planets, all of which give out heat. We know that heat is stored in the earth, and that it is in a very active condition we can perceive from the hot springs, lava, and flame which are continually erupting from the earth in various places. These sources of heat are beyond our control.

      But apart from the extra- and intra-terrestrial sources of heat there are mechanical causes for its generation upon our globe, such as friction, percussion, or compression. The savage or the woodman can procure heat and fire by rubbing a pointed stick in a grooved log. The wooden “breaks” of a locomotive are often set on fire by friction of the wheels, so they require grease, and the wheels on the rails will develop heat and sparks. Our matches, and many other common instances of the generation of heat (and fire) by friction, will occur to every reader. Water may be heated by shaking it in a bottle, taking care to wrap something round it to keep the warmth of the hand from the glass. By percussion, such as hammering a nail or piece of iron, the solid bar may be made “red-hot”; and when cannon are bored at Woolwich the shavings of steel are too hot to hold even if soap-and-water has been playing upon the boring-machine.

      The production of heat by chemical action is termed combustion, and this is the means by which all artificial heat for our daily wants is supplied. We can also produce heat by electricity. A familiar and not always pleasant instance of this is seen in the flash of lightning which will fuse metals, and experiment may do the same upon a smaller scale. These are, in brief, the Sources of Heat, and we may speak of its effects.

      We may take it for granted that no matter from what source heat is derived, it exhibits the same phenomena in its relation to objects. One of the most usual of these phenomena is expansion. Let us take water, and see the effect of heat upon it.

      We know that a certain weight of water under the same conditions has always the same volume; and although the attributes of the liquid vary under different circumstances, under the same conditions its properties are exactly the same. Now, water expands very much when under the influence of heat, like all liquids; solids and gases also expand upon the application of heat.

      We can easily establish these statements. A metallic ring when heated is larger than when cool. A small quantity of air in a bladder when heated will fill the bladder, and water will boil over the vessel, or expand into steam, and perhaps burst the boiler. So expansion is the tendency of what we term heat.

      We make use of this quality of heat in the thermometer, by which we can measure the temperature not only of liquids or solids, but of the atmosphere. The reading of the thermometer varies in different countries, for the degrees are differently marked, but the construction of the instrument is the same. It is called thermometer from two Greek words signifying the measure of heat. It is a notable fact that Castelli, writing in 1638, says to Ferdinand Cæsarina: “I remembered an experiment which Signor Galileo had shown me more than thirty-five years ago. He took a glass bottle about the size of a hen’s egg, the neck of which was two palms long, and as narrow as a straw. Having well heated the bulb in his hands, he placed its mouth in a vessel containing water, and withdrawing the heat of his hand from the bulb, the water instantly rose in the neck more than a palm above the level of the water in the vessel.”

      Here, then, we have an air-thermometer, but as it was affected by the pressure as well as the temperature of the atmosphere, it could not be relied upon as a “measurer of heat.” Until Torricelli propounded the principle of the barometer, this “weather-glass” of Galileo was used, for the philosopher divided the stem into divisions, and the air-thermometer served the purpose of our modern instruments.

      The actual inventor of the thermometer is not known. It has been attributed to Galileo, to Drebbel, and to Robert Fludd. There is little doubt, however, that Galileo and Drebbel were both acquainted with it, but whether either claimed the honour of the invention, whether they discovered it independently, or together, we cannot say. Sanctorio, of Padua, and Drebbel have also been credited with the invention. We may add that the spirit thermometer was invented in 1655–1656. It was a rough form of our present thermometer, and roughly graduated. But it was hermetically closed to the air, and a great improvement on the old “weather-glass. Edmond Halley introduced mercury as the liquid for the instrument in 1680. Otto von Guerike first suggested the freezing point of water as the lowest limit, and Renaldini, in 1694, proposed that the boiling and freezing points of water should be the limit of the scale.

      Let us now explain the construction and varied markings of the three kinds of thermometers in use. By noting the differences between the scales every reader will be able to read the records from foreign countries noted upon the Centigrade and Réaumur instruments, which are all based upon the theory that heat expands liquids.

      Fig. 79.—Thermometer.

      [We used to hear the expression, “Heat expands, and cold contracts,” but we trust that all our readers have now learnt that there is no such thing as cold. It is only a negative term. We feel things cold because they extract some warmth from our fingers, not because the substances have no heat.]

      Thermometers are made of very fine bore glass tubes. One end has a bowl, or bulb, the other is at first open. By heating the bowl the air in the tube is driven away by the open end, which is quickly dipped in a bowl of mercury. The mercury will then occupy a certain space in the tube; and if it be heated till the liquid boils, all the air will be driven out by the mercurial vapour. By once again dipping the tube in the quicksilver the glass will be filled. Then, before it cools, close the open end of the tube, and the thermometer is so far made. Having now caught our thermometer we must proceed to mark it, which is an easy process. By plunging the mercury into pounded melting ice we can get the freezing point, and boiling water will give us the boiling point. The intermediate scale can be then indicated.