or a liquid which has dissolved a quantity of gas, the more volatile component passes off, leaving the less volatile one behind. Thus, when a strong solution of hydrochloric acid in water is heated, the permanent gas at first passes off, leaving a weaker solution behind. In like manner, if chalk

be heated, the carbonic acid goes leaving lime behind.

off in the shape of gas,

On the other hand, many gases which are permanent by themselves may be brought into the liquid state, or condensed, in virtue of their strong affinity for certain liquids. Thus ammoniacal gas and hydrochloric acid gas have a great attraction for water, and if a jar of either of these gases be held above mercury, and a few drops of water introduced, the gas almost immediately disappears, being absorbed by the water. It is often very difficult to condense gases without making use of a solvent, and we have already stated that there are six which we have not yet been able to condense through the joint effect of cold and pressure-namely, oxygen, hydrogen, nitrogen, nitric oxide, carbonic oxide, and marsh gas.

212. Pressure of a Vapour in contact with its own Liquid.—We have seen that when a basin of liquid is allowed to evaporate under a receiver, vapour will rise from it until the vapour pressure in the receiver has reached a certain point, after which there will be no more evaporation. We have also seen that this point depends in the first place upon the nature of the liquid, and in the second place upon the temperature. The pressure thus attained is, in fact, the greatest vapour pressure possible for this particular liquid and temperature, and it is of importance to know for different liquids the maximum vapour pressure corresponding to various temperatures.

This information has been obtained by Regnault. We shall not attempt to describe the various and complicated apparatus which he made use of; rather let us state the most important results which he has obtained.

The following is an abridgment of his results for the vapour of water :

We are enabled to understand from this table how we may obtain the atmospheric pressure by observations of the boiling-point thermometer. In the first place, it is necessary

to bear in mind that in such instruments (Art. 168) the thermometer is not plunged into the water itself, but only into the vapour issuing from it; and in the next place, we must remember that when water boils (Art. 204) its vapour has the very same pressure as the atmosphere. Hence the rule is obvious. Look out on a table similar to the above the pressure corresponding to the reading of the boiling-point thermometer, and this will denote the atmospheric pressure.

Regnault has also ascertained the maximum pressures at various temperatures of other liquids besides water.

213. Density of Gases and Vapours. By means of Boyle's law we can ascertain how the density of a gas varies with its pressure, and, by means of Charles' law, how its density varies with its temperature. But in order to complete our knowledge of the subject we ought to know the density of various gases at a given temperature and pressure, say at the temperature of o° C. and the pressure of 760 millimetres of the mercurial column reduced to o° C.

Gay-Lussac was the first to discern that a connection subsists between the density of gases and their combining

chemical equivalents, and that when two gases combine together the volumes in which they combine bear a very simple relation to one another.

Thus, for instance, equal volumes of chlorine and hydrogen combine together without change of volume to form hydrochloric acid gas, which contains one atom of chlorine united to one of hydrogen. Equal volumes (at 760 millimetres and 'C.) contain, therefore, an equal number of atoms of these two gases.

Regnault has given us the following exact determinations of the weights of a litre of the most important gases :—

214. Recapitulation.—In what has gone before we have considered the effect of heat upon the volume and condition of bodies. We have seen that in general when the temperature of a solid rises it expands in volume, the rate of expansion being greater at a high temperature than at a low one. If sufficient heat be applied the body will pass from the solid to the liquid state, the change being in some cases very abrupt, but in other cases very gradual. In very many cases there will be an increase of volume accompanying this change of condition, but in some bodies there is, on the other hand, a contraction, ice being a notable instance of this latter class. When the liquid state has been completely assumed, the liquid generally increases in volume with any further increase of temperature, and at a greater rate than in solids, while the rate of increase is also greater at a high than at a low temperature. If the process of heating be still continued the liquid will pass into the gaseous form, and a very considerable expansion will take place. Finally, after the liquid

has been completely converted into gas, any further increase of temperature will augment the volume of this gas, the rate of increase being in general greater than in liquids or solids. 215. Effects of Heat upon other properties of Matter. In addition to those effects already mentioned there are many other ways in which this agent influences bodies. Thus we have

(1) The effect of Heat upon Refraction and Dispersion. Both of these diminish as the temperature increases.

(2) The effect of Heat upon the Electrical properties of bodies. This will be considered when we treat of Electricity.

(3) The effect of Heat upon Magnetism. This will be afterwards discussed when we treat of Magnetism.

Besides these there are other important effects. Thus, in most instances an increase of temperature promotes chemical combination, and when we speak of setting fire to a combustible substance, it is only another way of expressing the fact that a high temperature promotes combination. Occasionally, however, heat promotes decomposition, especially when one of the products of this decomposition is a gaseous body. Thus if limestone be heated lime will be left behind, and carbonic acid will be given off.

Again, the various phenomena of capillarity, such as capillary ascent and curvature, are affected by heat, becoming less marked when the temperature is high.

Extensibility, tenacity, and the various properties of solids are likewise affected by heat, and the compressibility of fluids is altered from the same cause. In fine, there is hardly a property of matter unaffected by this species of molecular motion.

LESSON XXIV.-CONDUCTION AND CONVECTION.

216. In the preceding pages we have described some of the most important effects of heat. Let us now consider the laws which regulate the distribution of heat through space.

In the first place, heat from a hot body, such as the sun or a star, proceeds outwards into a medium pervading all space,

in which it is propagated with very great velocity (Art. 106). It continues to proceed in the form of radiant heat until it reaches some body, such as our earth, by which it is absorbed, and it is in virtue of this process that we derive our heat from the sun. However, for the sake of convenience, we have agreed to regard radiant energy as a species of energy by itself, and we shall not therefore at present discuss the laws of radiant heat.

217. Conduction forms another well-known mode by which heat is distributed. If one end of a metal bar be thrust into the fire, and allowed to remain in it for some time, the other end will gradually become hot, until at length we shall be unable to touch it. The process by which heat is conveyed to the end of the metal rod is very different from radiation, for it is conveyed very slowly from particle to particle of the rod, until at length it affects that extremity which is farthest from the fire. But if instead of a metal rod we heat a glass or stoneware rod in the fire, the further extremity of this rod will never get very hot, because the substance of which it is formed does not conduct heat so well as a metal.

Organic fabrics, such as wool or feathers, form a still worse class of conductors; and this is the reason why these substances have been provided by nature as the clothing of animals, for the temperature of an animal is generally higher than that of the surrounding substances, and the heat is not readily conducted outwards through the garment of wool, feathers, or fur with which the animal is clad.

Liquids and gases are very bad conductors, but heat is distributed in them after a different manner, which we call Convection.

A bad conductor may be used, not only to keep in heat, but also to keep it out; for it may either be used to prevent the heat of the body being conducted outwards to those colder substances with which we are brought in contact, or if we wrap flannel round a block of ice it will, in virtue of its bad conducting power, prevent the heat from reaching the ice, and preserve it much better than another covering which might be a better conductor.