Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

The influence of the expansion of heat upon the specific gravity of liquids may be very prettily shown by the following experiment: Let some little bits of amber—a substance which is nearly of the same specific gravity with water—be thrown into water in a glass vessel, and let the water be heated, as represented in Fig. 194, by a spirit-lamp. That portion of the water which is heated passes upward because it is made specifically lighter, and colder water continually comes down to take its place. The upward and downward currents are as indicated by the arrows, the upward passing up in the middle, the downward coming down at the sides. This will be made manifest by the little bits of amber.

276. Thermometers.—It is the expansion of liquids by heat that in the thermometer gives us the measure of temperature. The liquid metal mercury is commonly used for this purpose, and answers well except in the extreme cold of the arctic regions. There, as mercury becomes solid at 39 degrees below zero, it is necessary to use a thermometer with alcohol in it, as this fluid can not be frozen by any degree of cold. The operation of the thermometer is simply this: Heat expands the fluid in the bulb, and the only way in which it can occupy more space as it expands is by rising in the tube. The abstraction of heat, on the other hand, causes contraction, and of course a proportionate falling of the fluid.

Fig. 195.

277. Fahrenheit's Thermometer.—The thermometer was invented in the beginning of the seventeenth century, but it is not decided who was the inventor. There may have been in this case, as in others, more inventors than one, the same ideas having, perhaps, entered several inquiring minds at the same time. Various fluids were used by different persons. Sir Isaac Newton used linseed oil. Fahrenheit, a native of Hamburg, who flourished in the first part of the last century, was the first to use mercury. Though various propositions were made by Newton and others in regard to the measurement of heat by thermometers, no thermometric scale seems to have met with general reception till that of Fahrenheit's, which was put forth about 1720. The plan of it is this: His zero is the point at which the mercury stood in the coldest freezing mixture that he could make; and he supposed that this was the greatest possible degree of cold, as it was the greatest that he knew. He next found the point at which the mercury stood in melting ice. This he called the freezing point, because the temperature is the same in water passing into the solid from the fluid state as in water passing into the fluid state from the solid. In other words, this point in the scale marks the transition line between the two states. From this point Fahrenheit marked off 32 equal spaces or degrees down to zero. He now found the point at which the mercury stands in boiling water, and called this the boiling point. Marking off the space on the scale between this, and the freezing point in the same manner, there are 180 degrees—that is, the boiling point is 212 degrees above zero. The degrees above zero are commonly designated by the mark +, plus; and those below by the mark -, minus. Thus, +32° signifies 32 degrees above zero, and -32° signifies 32 degrees below.

278. Other Thermometers.—Fahrenheit's thermometer is the one commonly used in this country. But there are several other thermometers on different scales, as the Centigrade, Reaumur's, and De Lisle's. In Fig. 195 you see the plans of the scales of these thermometers placed side by side. In the Centigrade thermometer, which is in use in France, and indeed in a large part of Europe, the zero, you see, is placed at the freezing point; and the space between this and the boiling point is divided into 100 degrees, which gives it the name Centigrade. Reaumur's, which is in use in Russia, has the same zero, but he has only 80 degrees from this to the boiling point. De Lisle's, which has gone entirely out of use, has its zero at the boiling point. The arrangement of Fahrenheit, although its zero is a mere arbitrary point, is, on the whole, the best, because its degrees are of such a size that they mark differences of temperature with sufficient minuteness for all practical purposes of an ordinary character without resorting to fractional parts.

Fig. 196.

279. Expansion in Aeriform Substances.—Heat produces a vastly greater expansive effect in air, the gases, and vapors, than it does in liquids. The expansion of air by heat may be shown very prettily in this way: Take a glass tube that has a bulb on one end, and, placing the other open end in water (as represented in Fig. 196), apply the palm of your hand to the bulb. The heat of the hand being communicated to the bulb will expand the air, and so, as you see, bubbles of air will escape through the water. On removing the hand, and allowing the bulb to cool, the air in it will be condensed, and water will pass up in the tube in proportion to the amount of air which has escaped. A bladder partly filled with air will be made to swell out to plumpness if it be heated sufficiently, and a full one may be so heated as to burst from the expansion of the air. Porous wood, as chestnut, snaps very much when burned, because the heat expands the air contained in the pores.

280. Balloons.—The first balloons that were used were filled with heated air. You have already seen, in § 149, why it is that balloons rise. Now in the hot-air balloon it is the expansion of the air by heat that makes it lighter than the surrounding air. Of course such a balloon is not as effective as the gas balloon, for the air within it loses its comparative lightness as it becomes cooled; while the gas which is used, being very much lighter than air at the same temperature, does not lose its lightness as the balloon goes up. You learned in § 152 that the atmosphere becomes thinner as we go upward. The gas balloon, therefore, rises until it arrives at that point where the air is of about the same specific gravity with the gas, and there it stops. It is made to descend by letting out some of the gas from a valve. Gas was not used for balloons till 1782. Hydrogen gas was employed at first, being over fourteen times lighter than air. Of late the common burning gas, carbureted hydrogen, has been generally used, because it can be so readily obtained where there are gas-works.