460

Expansion of Gases.

it will go on increasing in volume up to 47°; if we cool the water from 39° to 32° it goes on expanding in equal proportion until it freezes. Water at 47° and 32° are therefore equal in bulk. This curious fact may be illustrated by the following experiment :-Place glass bulbs provided with stalks containing water and mercury, and graduated from 32° to 50°, in basins in which ice is melting, The mercury will sink down to 32° and there remain steadily. The water will sink until it reaches 39° (or more correctly 391°); but it will then begin to rise, although still undergoing the cooling process. When just about to freeze, i.e., at 32°, the water will have expanded so as to mark 47°. Thus, whether cooled 7° below 39° or heated 7° above it, it will occupy the same volume. Hence it appears that there is an expansion of water by cold as well as by heat, a fact for which no theory of heat has yet satisfactorily accounted.

The temperature of maximum density (39°) applies only to fresh water. If it contains much saline matter, the degree for maximum density is much lowered. Thus sea water continues to decrease in bulk down to its freezing point, which is about 27° if the water is agitated, and 25° degrees if still. It is just before it freezes that salt water has its greatest density. This is indicated by the specific gravity imparted by the salt, and not by temperature. This will explain why in the soundings taken in the deep sea the temperature of the bottom stratum is not invariably found at 391° or 40°.

666. Expansion of Gases.—Gases are expanded by heat still more than liquids. A difference of one degree causes a perceptible difference in volume, and allowance must be made for this in the measurement of a gas. Heated air weighs less than cold air, a fact which may be thus demonstrated :-Balance two cones of paper with the wide open ends downwards at the end of a scale beam. The cones may be easily made of cartridge paper cut to shape and gummed. The temperature of the air is the same in the two cones, and they will be exactly equal in weight. If a lighted wax taper is now introduced under one of the cones the air in it is expanded, a portion is forced out, and the cone now rises. By transferring the taper to the other cone, the air in that may be rendered lighter-the experiment being performed alternately with each cone, allowing sufficient time for cooling.* The ascent of the Montgolfier or fire-balloon is based entirely on this principle.

The extent of this dilatation in gases is so much greater than * A balanced thin glass shade may be substituted for the paper cones

with a similar result.

Effects of Gaseous Expansion.

461 in liquids or solids, that it forces itself much more strikingly upon the attention. Thus a bladder containing a small quantity of air and secured by a stop-cock, becomes apparently filled and quite tense on being held to the fire. The air in a balloon just escaping from a cloud, has been so suddenly expanded by the direct rays of the sun, as to injure the texture of the balloon. Some of the fatal accidents among aëronauts have been owing to this occurrence.

In consequence of this great increase of volume by heat, hot air readily floats on cold, a point of considerable importance in reference to warming and ventilation. During cold weather thermometers placed on the floor and the ceiling of an apartment heated by an open grate, will indicate very different degrees of temperature. In the opening of a door the flame of a candle will be carried outwards at the top by the warm air rushing out of the room, while on the floor, it will be blown inwards by the current of cold air flowing in.

The expansion of gaseous or aëriform bodies by heat produces many important effects in nature. Some of these have already been considered in preceding parts of this work, as, the rising of heated air in the atmosphere causing the winds all over the earth; the same in our fires and chimneys supporting combustion, and ventilating and purifying our houses; the same again from around animal bodies, removing the poisonous or contaminated air which issues from the lungs, and insuring a constant supply of fresh air for the support of life.

The expansion of air and other gaseous matter by heat has lately become a subject of very high interest, from having led to new views as to the nature not only of heat, but of force or energy in general. This subject will be considered in another place.

"The expansion of bodies by heat increases more rapidly than the temperature, and particularly near the melting and boiling points, that is, their points of changing into liquid or gas."

667. If a certain increase of temperature, accurately measured by any of the methods now practised, be given to a mass of cold water, it will produce in that a certain increment of bulk and if other equal additions be afterwards successively made, each will produce a rather greater increment of bulk than the preceding, with diminished specific gravity, particularly when the water approaches to boiling. Thus, 90° added to water at 32° produces a

462

Unequal Expansion of Liquids.

certain expansion or increase of volume amounting to 4°7 in raising it to 122°. When, however, 90° are added to the liquid already at a temperature of 122°, so as to raise it to 212°, the rate of expansion is 15, or nearly threefold that which was produced by the same number of degrees at the lower temperature. It is this inequality which renders water wholly unfitted for the purposes of a thermometer. It is found that after the water has been converted into steam, or become aëriform, any farther increase of bulk is always closely proportioned to the increase of temperature. What is thus true of water in relation to heat is true of bodies generally, each, however, having a rate of expansion and temperatures for melting and boiling proper to itself. The quickened rate of expansion ir solids and liquids might have been anticipated, from reflecting that cach successive quantity of heat added to a liquid, meets with less resistance to its expanding power than the preceding quantity, owing to the diminishing force of the mutual attraction of the particles as they separate from each other; while in a gas, as such cohesion has altogether ceased, each addition of heat is at liberty to produce its full effect. If the capacity of substances for heat did not increase with their bulk, the terms “increase in the amount of heat" and "increase of temperature" would have the same meaning, and this subject would be more simple.

668. The reflection may naturally occur here, that, as in the common thermometer, the mercury must rise or expand more for a given quantity of heat added at a high than at a low temperature, the scale should be so divided as to correspond with the inequality. This reasoning is good, but the difficulty of complying with it in practice is such, that the inconvenience of the slight error arising from an equal division is commonly submitted to. An air-thermometer having equal divisions is more nearly correct, but from wanting many of the advantages of the mercurial thermometer is little employed. The subject of unequal thermometric dilatation in the same liquid, and of the differences in that respect in different liquids, depending on the proximity to their boiling points, was well illustrated by De Luc's experiment of charging with different liquids, thermometer-tubes divided according to the scale of Reaumur, and, while they were being heated through the same range of temperature, from his zero (0°) or freezing point to boiling So°), noting their comparative indications. The discordance of the dilatations in different tubes when the instruments were placed together and heated from the freezing (marked o°, or zero, cn

Reaumur's scale) to the boiling degree of water (marked 80°), was as here detailed

The singular discrepancy in the case of water is owing to the peculiarity, described in Art. 665, of its contracting by cold only down to about 40° of Fahrenheit, and then again dilating until it freezes.

"To melt a solid body, or to vaporize a liquid, a large addition of heat enters into it, but in the new arrangement of the particles and the generally increased volume of the mass, the heat becomes hidden from the thermometer and is called LATENT HEAT. It may be made to re-appear during the converse changes, after any interval whatever."

669. The expansion of bodies by heat, instead of proceeding throughout in a nearly uniform or gradual manner, makes in its course two great leaps, with singular transformations of the body: the first, when the solid breaks down into a liquid ; the second, when the liquid expands into a gas; so that there are in all three very distinct modifications or stages of existence for the body, dependent on the agency of heat. Water, for instance, when at a low temperature, exists in the solid form called ice; but at 32° of Fahrenheit, on receiving more heat, it gradually becomes liquid or water; and on receiving still more heat it acquires at 212°, even under the resisting pressure of the atmosphere, a bulk nearly 2000 times greater than it had as a liquid (gradually as regards the whole, but suddenly as regards each separate portion), being then called steam, or aëriform water, or aqueous vapour. Other bodies under analogous circumstances undergo similar changes. It is further remarkable, that although during the changes a large quantity of heat enters the

464

Experiments on Latent Heat.

mass, producing in the one case liquidity, in the other the form of gas or vapour, the temperature or indication of the thermometer is the same, immediately after, as immediately before the change, the heat received in the interval becoming hidden or latent in the mass :—thus water running from melting ice affects the thermometer just as the ice does, and steam over boiling water appears no hotter than the water. The glory of originally discovering the facts, to recall which the terms latent heat are used, belongs to the illustrious Dr. Black. No discovery in reference to heat has proved of greater importance to mankind than this. The modern steamengine was an early result of this discovery and of kindred investigations made by his friend, James Watt.

670. We may select the following instances as serving to display the subject of latent heat in its various bearings.

A mass of ice brought into a warm room, and there receiving heat from every object around it, will soon reach the temperature of melting or 32°, but afterwards both the ice and the water formed from it will continue at that temperature until all be melted. The heat which continues to enter the solid effects a change only in the form, not in the temperature of the mass. The temperature of the liquid is not raised in the smallest degree. It remains at 32° until all the ice is melted. By this invariable result one may test the accuracy of a thermometer. Whatever time may have been required for heating the mass of ice one degree, just one hundred and forty times as much will be required for melting it; proving that 140° is the latent heat of water.

If two similar flasks, one filled with ice at 32°, and the other with water at 32°, be placed in the same oven or over like flames, the water will gain 140 degrees of heat, while the ice is merely being melted into water at 32° : and in the course of the experiment, a correspondence will always exist between the phenomena; for instance, when the water has gained 14° of heat, it will be found that just a tenth part of the ice is melted.

If equal quantities of hot and cold water be mixed together, the whole acquires a middle temperature, each degree lost by the hot water becoming a degree gained by the cold. Thus, on mixing equal measures of water at 70° and 130°, the mixture will have the mean temperature of 100°, the hot water loses 30°, and the cooler water gains 30°. Hence it follows that if equal weights of water at 32° and 172° respectively are mixed, the temperature of the mixture will be 102°. But if ice at 32° be mixed with an equal weight of