melted rock or lava from active volcanoes. The existence of natural hot-springs points to the same conclusion. Further, on descending deep mines, the temperature is found to increase continually with the depth; and the water from very deep wells is found to be hotter than that from shallower wells. On the whole, the earth has not sensibly decreased or increased in temperature within human records.

11. Friction. When the surfaces of any two substances whatever move over one another in contact, heat is invariably given out by both. The quantity of heat produced by friction varies with the labour expended in effecting motion-that is, with the amount of friction. Rough solids having greater friction than smooth ones, greater heat is produced by their motion upon one another, other things being equal. For the same reason, less heat is produced by the friction of liquids than by that of solids, and still less by the friction of gases. Examples of heat, caused by the friction of solids, are found in the lighting of a match, the warmth produced by rubbing the hands, the scorching and ignition of a piece of hard dry wood when it is rubbed violently on a soft board, the burning of the edge of a boat by the rapid motion over it of a whale-rope. The heat generated by the friction of iron disks has been employed for heating houses. A rod of iron may be made hot enough to ignite wood by quick and heavy hammering; this effect is chiefly due to the friction of the parts of the iron on one another. Ice may be melted by rubbing two pieces together.

That the friction of liquids produces heat, is seen from the fact that still water freezes more readily than that which is agitated. The temperature of the water in a bottle is sensibly raised on shaking it violently. The warmth of animals is partly caused by the friction of the blood in their arteries and veins.

The wind gives rise to heat by its friction upon itself and upon the solids and liquids it meets with. The temperature of a dry solid may be sensibly raised by whirling it rapidly through the air. Aërolites become red-hot on entering and tearing through the atmosphere.

H

Fig. 2.

§ 12. Change of density by pressure.-Whenever a fixed quantity of gas undergoes diminution of volume by pressure, heat is evolved. Thus, if C (fig. 2) be a cylinder containing a fragment (T) of German tinder, and if a close-fitting piston (P) be inserted at the top and forcibly and rapidly pressed down towards T by means of the handle and rod H, the original volume of air in the cylinder will be compressed into the space between the bottom of the piston and the bottom of the cylinder. As the air becomes thus more and more compressed, it gets hotter and hotter, until at last may set fire to the tinder.

it

The barrel of an air-gun becomes hot as the air is pumped into it.

It is probable that solids and liquids also give out heat when mechanically compressed. Their change of volume is, however, so slight, under the greatest attainable pressures, that the heat, if any, liberated is inappreciable.

C

§ 13. Change of physical state.—When a vapour is condensed into a liquid by pressure or other means, heat is given off. Thus steam, at the temperature of boiling water, if forcibly compressed, condenses into water, having a higher temperature. Gases, when they dissolve in water or other liquids, thereby themselves becoming liquid, give out heat.

Liquids, when they become solid, give out heat. With some precautions, many salts may be dissolved in water in such quantities that, on the slightest agitation, the salt crystallizes out; in doing so it becomes warm. Water itself, if kept quite still, may be cooled below the temperature at which it ordinarily freezes; when disturbed, part of it becomes ice and, in doing so, rises in temperature. This source of heat will be further considered in §§ 96–137, under liquefaction, ebullition, evaporation.

The following diagram shows the relation of heat to the three forms of matter-solid, liquid, and gaseous,-the direction of the arrows denoting the kind of change of physical state.

§14. Chemical change.-When two different kinds of matter are brought into contact, they often interpenetrate and unite in a peculiarly intimate manner, giving rise to a product different from both. This action is called chemical union; its consideration forms a main portion of Part II. Chemical union is always attended by the liberation of heat. The solid quicklime, in contact with the liquid water, is converted into slaked lime, becoming hot. When heated together, a part of the air (a gas) unites with carbon (a solid), giving rise to a heavy irrespirable gas, and producing fire. The same part of the air may unite with fat, wax, wood, paper, spirits of wine, &c., giving rise to the same gas, together with vapour of water, and producing flame. The heat of fermentation, putrefaction, and decay is also caused by chemical change.

Chemical change, of which burning or combustion is the most common instance, furnishes most of the heat produced artificially for the purposes of life.

§ 15. Electrical discharge, both of frictional or high-tension and of voltaic or low-tension electricity, gives rise to heat. Instances of the former are seen in the burning or fusion of bodies by lightning, and the burning or volatilization of thin wire by the artificial spark. Instances of the latter occur when a voltaic circuit is completed or broken; when it has to traverse a substance of conducting-power insufficient to allow it to pass freely (a narrow wire), or when a strong current has to leap over a short interval (the heat of the electric light).

§ 16. Living beings are generally of a higher temperature than the surrounding media. This is sometimes attributed to what is called Vital heat. Such heat is probably mainly due to the chemical changes and electrical discharges taking place in the tissues, and to the friction of the juices.

CHAPTER III.

EXPANSION.

§ 17. Almost all bodies, whatever be their temperature, and whether they be solids, liquids, or gases, expand or increase in volume when they increase in temperature. This is supposed to be due to the heat, on entering the body, separating its particles further from one another than they were when the body was cooler.

§18. Expansion of gases by heat.-All gases, at all temperatures, expand equally for equal increases of temperature*. Thus, if cubic feet of all known gases, at the temperature of melting ice, be heated to the temperature of boiling water, they all increase 100

equally in bulk, each one becoming 1+273 cubic foot—that is, 1.366 cubic foot. On cooling them to the original temperature, they all shrink equally, resuming their original volume of 1 cubic foot.

Further, if they all be heated to any the same intermediate temperature, they will all have the same size, intermediate between 1 and 1.366 cubic foot.

Again, if they all be heated to a temperature exactly halfway between those of melting ice and boiling water, they will all

have exactly the same volume (1+·

.366
2

=1.183 cubic foot). And

if they be made as much hotter than boiling water as boiling water is hotter than melting ice, they will all have the volume 1+2× 366=1.732 cubic foot.

In general terms, if we suppose the range of temperature between melting ice and boiling water to be divided into 100 equal parts or "degrees," the increase of volume which a given volume of the gas undergoes (on being heated through one of

* This law is not of absolute truth under all circumstances, but quite sufficiently so for all ordinary purposes.

1

those degrees) is

100

of its increase when heated through 100 of them; and its increase on being heated through n of such degrees

n

is of its increase when heated through 100 of them; that is, 100

n of '366, or n×·00366. The entire volume of the heated gas

100

1

is therefore its original volume at the lower temperature, together with this increase; or if V1 be the original volume at the lower temperature, and V, be the volume at a temperature t degrees higher, then

2

Example 1.-20 cubic centimetres of air at 12 degrees are heated to 26 degrees; required the volume of the warmer air.

Here V1=20 cub. centims., t=26—12=14;

.. V, 20 cub. centims. +20 x 14 x .00366

=

=21.0248 cub. centims.

Example 2.-48 cubic centimetres of hydrogen at 51 degrees are cooled to 17 degrees; required the volume of the cooler gas. Here V, 48 cub. centims., t=51-17=34;

§ 19. It is to be remembered that if the quantity or weight of a substance remains the same, its density varies inversely as its volume. We know also, by Boyle and Mariotte's law, that the volume of a gas varies inversely as the pressure to which it is exposed. (See Pneumatics.) We are now, therefore, able to