has been observed, and by artificial means the temperature can be lowered to -148° F. (-100° C.), or even lower. When great degrees of cold are to be measured, alcohol is used in the construction of this instrument, as it does not congeal at 148° F. (-100° C.).

Mercury boils at 662° F. (350° C.); therefore its use must be limited to temperatures below this point. The high temperatures attending ignition are sometimes measured by the expansion of bars of platinum, a metal which does not melt even in the hottest furnace. Such an instrument is called a pyrometer. The determination of very high temperatures is, however, still very unsatisfactory.

Expansion of Solids.-If an iron vessel, when cold, is just large enough to pass through the door of a furnace, it cannot be removed from it when heated. The iron bands or tires of carriage-wheels are applied while red-hot to the frame, and on cooling they contract and bind the wood-work together with great force. A metallic disk, which, when redhot, fits exactly into a circular box, will, on cooling, become loose, and shake in it. The tire and the disk both become smaller on cooling. These examples show that solids also are expanded by heat and contracted by cold, and explain many of the phenomena of common life. Clocks are apt to go faster in winter, and slower in summer, because the pendulums elongate in summer, and, consequently, vibrate slower; while in winter they become shorter, and vibrate more rapidly. A piano gives a higher tone in a cold than in a warm room, on account of the contraction of the strings. A nail driven into the wall becomes loose after a time, because the iron expands in summer and contracts in winter more than the stone or the wood, and thus the opening is gradually enlarged. For this reason, in the construction of railroads, the rails must not be laid too close together; in the arrangement of steam-pipes, these must not be too firmly inclosed; in roofing, the zinc plates, instead of being nailed together, must overlap each other, that they may neither tear nor warp on alternate contraction and expansion.

Brittle bodies, as glass and porcelain, expand or contract so rapidly, by sudden heating or cooling, that they break.

Experiment 3.-Wind round a vial two bands of paper, a and b, Fig. 12, and secure them firmly with thread; pass a

piece of string round the vial, between these folds of paper, and move the vial quickly to and fro on the string until the latter breaks. Then immediately pour cold water upon the place, and the glass will break as evenly as if cut. The sharp edges can be removed with a file. In this manner, common white or green glass bottles may be converted into vessels adapted to chemical and other purposes.

Fig. 12,

It is well known that heat is produced by the friction of two bodies upon each other; that by sliding quickly down a line or a pole by the hands, these will be burnt; and that rapid motion will ignite the axles of a carriage, unless they are well greased. Thus, in the above experiment, the friction produced great heat in the glass, the string emitted a burnt odour and broke, and great expansion of the glass was produced. When the outer surface was suddenly cooled by the cold water, the expanded particles at once contracted, and more rapidly in the external particles than in those of the inner surface, causing the fracture of the glass, and the more easily the greater its thickness. If the temperature had been slowly reduced, it would not have broken.

Thus it is obvious (a) that glass and porcelain vessels intended for sustaining high temperatures, such as flasks, alembics, retorts, capsules, &c., should be thin, particularly at the bottom; and (b) that, when used, they should always be gradually heated and cooled.

The above method of heating glass by a piece of string furnishes the chemist with a simple expedient for removing stoppers which are too firmly fixed in the bottles to be taken out by turning or tapping them. All that is necessary is to wind a piece of thick string round the neck of the bottle, and move it quickly until sufficient heat has been produced to loosen the stopper.

No two solids expand alike; the metals expand the most, and all solids less than fluids.

Expansion by Cold.-A remarkable exception to this law of expansion by heat and contraction by cold occurs in the case of water.

Fig. 13.

Experiment 4.-A large flask is arranged as directed in Experiment 2, page 29, but inserting a cylindrical thermometer, a, through a second hole made in the cork. The flask is filled with water to the top of the tube b, and placed in a vessel filled with snow. A strip of paper may be pasted on this tube, upon which the level of the water may be marked as the thermometer falls. The water as it cools will sink in the tube until the mercury stands at 39° F. (4° C.); yet, on cooling still more, it does not fall any further, as we should expect it would, but, on the contrary, it begins to rise again, and continues to do so till it reaches the freezing point. At 32° F. (0° C.) it stands at the same point as when its temperature was at

46° F. (8° C.). Water is accordingly most dense at 39° F. (4° C.); all other liquids continue to increase in density as they cool.

However unimportant this exception may appear at first, our admiration must be the greater when we reflect upon its consequences. Were it not for this, our country would have the climate of Greenland. The freezing of our waters, as the winter sets in, is principally owing to the coldness of the atmosphere. Consequently, the upper part of the water is colder and heavier, and sinks to the bottom; the warmer water ascends, becomes cold, and also sinks. If the water continually became denser, to its freezing point, this circulation would continue till the whole mass of water to its greatest depth reached 32° F. (0° C.), and a few cold days would suffice to convert all our ponds, lakes, and rivers into ice. This does not happen, because the circulation ceases when its temperature has fallen to 39° F. (4° C.), when the water, though yet colder, becomes lighter, and floats on the surface. Thus, freezing can only take place at the surface, and the ice be but gradually formed. At a small depth below the ice the water generally retains the temperature of 39° F. (4° C.)

Expansion of Gases.-Experiment 5.-Dip a glass tube, pro

D

Fig. 14.

vided with a bulb, into water, and heat the bulb gently; a part of the air is expelled, and escapes in bubbles through the water; consequently, there is not room enough in the bulb for the heated air; but it requires a larger space than it did in its cold condition. It follows from this, also, that the warm air is lighter than cold. If the lamp be removed, the air remaining in the bulb will contract on cooling, and water will be pressed up into the bulb, replacing the air which has been expelled. Such an apparatus, when the tube is small, may be used as an air thermometer. When the bulb is heated, the liquid descends in the tube, and vice versa.

All gases expand alike when equally heated, differing in this respect from solids and liquids. The extent to which they expand is moreover much greater than with either of the other forms of matter. A gas at freezing point expands 49 of its volume for every degree Fahrenheit (23 of its volume for every degree centigrade) that is added to its temperature, so that it is very easy, knowing the volume that a certain weight of gas occupies at one temperature, to calculate what its volume would be at some given standard temperature. In experiments on gases their volume is always given (unless otherwise stated) at 0° C. or 32° F., which is therefore said to be the standard of temperature.

Example.-A certain weight of gas measures 23 cubic inches at 65° F.: what would be its volume at 32° F.?

491 volumes at 32° become 492 at 33°, 493 at 34°, and so on, increasing 1 volume for every 1° F. Therefore, at 65°, the 491 volumes would become 491 + 33 = 524 volumes. Hence the calculation is a mere rule of three sum:-

524 : 491 :: 23 x 21.5 cubic inches. Similar calculations may of course be made in the centigrade scale, as the following example will show.

We have 100 measures of a gas at 50° C.: what would be its volume at 0° C.?

273 vols. of gas at 0° C. 274 vols. at 1°, 275 at 2°, and

so on.

Heated from 0° to 50°, 273 vols. would become 323 vols. Therefore

323: 273 :: 100: x = 84.5 measures.

We can in this way find the volume that a gas would occupy at any given temperature, although it most frequently happens that we want to find its volume at freezing point. We have 100 volumes of gas at 17° C.: what would be its volume at 193° C. ?

273 vols. at 0° C. Therefore

193°.

=

290 vols. at 170, and 466 vols. at

290: 466 :: 100 : x = 160 7 volumes. These calculations are generally spoken of as for temperature."

Change of State as produced by Heat.

66 corrections

(1) From Solid to Liquid.-Melting of Solids.-Expansion is the first general effect of heat; but in many solid bodies another effect is observed; they change their state of aggregation, they become liquid, they melt. Many of them become soft before melting, so that they can be kneaded; for instance, butter, glass, and iron; in this condition, glass can be bent and moulded like wax, and iron can be forged.

Experiment 1.-Hold a piece of a small glass tube in the upper part of the flame of a spirit-lamp, revolving it slowly between the fingers; when red-hot, it will be so soft that it can be bent into any shape desired. Thus are easily formed any of the numerous bent tubes required in chemical experiments. For softening larger tubes, a lamp with a double current of air or a Bunsen burner must be used, as this gives a much stronger heat than the simple lamp. To break a glass tube, a scratch is made upon it with a three-cornered file, at the place to be broken, and then it can be parted by gently bending it with both hands.

Most solid bodies become suddenly fluid, as ice, lead, &c. Experiment 2.-Place one vessel containing snow or ice, and another containing a piece of a tallow-candle, on a warm stove, dipping a thermometer from time to time into the melting substances; the temperature will remain stationary in the first vessel at 32° F. (0° C.), in the other at about