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In theatres the spectators in the galleries are exposed to the highest temperature and the most impure air, while those near the orchestra respire in a purer air.

Draughts in chimneys are due to the expansion of air. Heated by the fire in the grate, the air rises in the chimney with a velocity which is greater the more it is expanded. Hence results a rapid current of air, which supports and quickens the combustion by constantly renewing the oxygen absorbed.

The expansion and contraction of air have a fortunate influence on the temperature of that part of the atmosphere in which we live. For when the ground is strongly heated by the sun's ardent rays, the layers of air in immediate contact with it tend to acquire the same temperature and to form a stifling atmosphere; but these layers, gradually expanding, rise in virtue of their diminished density; while the higher layers, which are colder and denser, gradually replace them. Thus the high temperature which would otherwise be produced in the lower regions is moderated, and never exceeds the limits which plants and animals can support.

The expansion and contraction produced in the atmosphere over a large tract of country are the cause of all winds, from the lightest zephyr to the most violent hurricane. These winds, which at times are so destructive, so capricious in their direction, and so variable in their intensity, not merely have the effect of mixing the heated and the cooler parts of the atmosphere, and of thus moderating extremes of temperature, but by driving away the vitiated atmosphere of our towns, and replacing it by pure air, they are one of the principal causes of salubrity; without them our cities would be centres of infection, where epidemics of all kinds would be permanent. Without winds clouds would remain motionless over the country where they were formed, the greater part of the globe would be condemned to absolute aridity, and neither rivers nor brooks would moisten the soil. But carried by the winds the clouds formed above the seas are transported to the centres of continents, where they fall as rain; and this having fertilised the soil, gives rise to the numerous rivers which fall into the ocean, thus establishing a continuous circulation from the seas towards the continents and from continents towards seas.

218. Density of gases.-The densities of solids and of liquids have been determined in reference to water (100); those of gases by comparison with air; that is, having taken as term of comparison, or unity, the weight of a certain volume of air, the weight

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Fusion.

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of the same volume of other gases is determined. But as gases are very compressible and very expansible, and as therefore their densities may greatly vary, they must be reduced to definite pressure and temperature. This is why the temperature of zero and the pressure 30 inches have been chosen.

Hence the relative density of a gas, or its specific gravity, is the relation of the weight of a certain volume of the gas to that of the same volume of air; both the gas and the air being at zero and at a pressure of 30 inches.

In order, therefore, to find the specific gravity of a gas, oxygen for instance, it is necessary to determine the weight of a certain volume of this gas, at a pressure of 30 inches, and a temperature of zero, and then the weight of the same volume of air under the same conditions. For this purpose a large globe of about two gallons capacity is used, like that represented in fig. 88, the neck of which is provided with a stopcock, which can be screwed to the air-pump. The globe is first weighed empty, and then full of air, and afterwards full of the gas in question. The weights of the gas and of the air are obtained by subtracting the weight of the exhausted globe from the weight of the globes filled, respectively, with air and gas. The quotient, obtained by dividing the latter by the former, gives the specific gravity of the gas. It is difficult to make these determinations at the same temperature and pressure, and therefore all the weights are reduced by calculation to zero, and the normal pressure of 30 inches.

In this manner the following densities have been found :

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From these numbers the lightest of gases, and therefore of all bodies, is hydrogen, whose density is less than 1th of air.

CHAPTER VI.

CHANGES OF STATE OF BODIES BY THE ACTION OF HEAT.

219. Fusion. In treating of the general effects of heat, we have seen that its action is not only to expand them, but to cause them to pass from the solid to the liquid state; or from the latter state

to the former, according as the temperature rises or falls; then from the liquid to the aëriform state, or conversely. These various changes of state we shall now investigate under the name of fusion, solidification, vaporisation, and liquefaction.

Fusion is the passage of a solid body to the liquid state by the action of heat. This phenomenon is produced when the force of cohesion which unites the molecules is balanced by the force of repulsion (4); but as the cohesive force varies in different substances, the temperature at which bodies melt does so likewise. For some substances this temperature is very low, and for others very high, as the following table shows :

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Some substances, however, such as paper, wood, wool, and certain salts, do not fuse at a high temperature, but are decomposed. Many bodies have long been considered refractory; that is, incapable of fusion; but, in proportion as it has been possible to produce higher temperatures, their number has diminished. Gaudin has succeeded in fusing rock crystal by means of a lamp fed by a jet of oxygen; and more recently Despretz, by combining the effects of the sun, the voltaic battery, and the oxy-hydrogen blowpipe, has melted alumina and magnesia, and softened carbon, so as to be flexible, which is a condition near that of fusion.

Some substances pass from the solid to the liquid state without showing any definite melting point; for example, glass and iron become gradually softer and softer when heated, and pass by imperceptible stages from the solid to the liquid condition. This intermediate condition is spoken of as the state of vitreous fusion. Such substances may be said to melt at the lowest temperature at which perceptible softening occurs, and to be fully melted when the further elevation of temperature does not make them more fluid; but no precise temperatures can be given as their melting points.

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Latent Heat.

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220. Laws of fusion.-It has been experimentally found, that the fusion of bodies is governed by the two following laws :

I. Every substance begins to fuse at a certain temperature, which is invariable for each substance if the pressure be constant.

II. Whatever be the intensity of the source of heat, from the moment fusion commences, the temperature of the body ceases to rise, and remains constant until the fusion is complete.

For instance, the melting point of ice is zero, and a piece of this substance exposed to the sun's rays, placed in front of a fire or over a lamp, could never be heated beyond this temperature. Exposure to a more intense heat would only accelerate the fusion, the temperature would remain at zero until the whole of the ice was melted.

221. Latent heat.—Since, during the passage of a body from the solid to the liquid state, the temperature remains constant until the fusion is complete, whatever be the intensity of the source of heat, it must be concluded that, in changing their condition, bodies absorb a considerable amount of heat, the only effect of which is to maintain them in the liquid state. This heat, which is not indicated by the thermometer, is called latent heat, or latent heat by fusion, an expression which, though not in strict accordance with modern ideas, is convenient from the fact of its universal recognition and employment.

An idea of what is meant by latent heat may be obtained from the following experiment. If a pound of water at 80° is mixed with a pound of water at zero, the temperature of the mixture is 40°. But if a pound of pounded ice at zero is mixed with a pound of water at 80°, the ice melts, and two pounds of water at zero are obtained. The pound of ice at zero is changed into a pound of water also at zero, but as the hot water is also lowered to this temperature, what has become of the 80° of heat it possessed? They exist in the water which results from the ice; their effect has neither been to heat it nor to expand, but simply to impart fluidity to it. Consequently, the mere change of a pound of ice to a pound of water at the same temperature requires as much heat as will raise a pound of water through 80°. This quantity of heat represents the latent heat of the fusion of ice, or the latent heat of water. Every substance in melting absorbs a certain amount of heat, which, however, varies materially.

The enormous quantity of heat absorbed by ice in melting explains how it is that so long a time is required for thaw. And conversely it is owing to the latent heat of water, that even when

its temperature has been reduced to zero so long a time is required before it is entirely frozen. Before it can be so it must give out the heat which had been consumed in its liquefaction: it thus becomes a source of heat which retards the solidification. Faraday has calculated that the heat given out by a cubic yard of water in freezing is equal to that which would be produced by the combustion of a bushel of coals.

222. Solidification.—Those substances which are liquefied by heat revert to the solid state on cooling, and this passage from the liquid to the solid state is called solidification. If this solidification takes place at a low temperature it is frequently spoken of as congelation.

In all cases the phenomenon is subject to the following laws : I. Every body, under the same pressure, solidifies at a fixed temperature, which is the same as that of fusion.

II. From the commencement to the end of the solidification, the temperature of a liquid remains constant.

Thus if lead begins to melt at 335°, melted lead in like manner when cooled down begins to solidify at 335°. Moreover, until it is completely solidified, the temperature remains constant at 335°. This arises from the fact, that the liquid in proportion as it solidifies restores the heat it had absorbed in being melted. The same phenomenon is observed whenever a liquid solidifies.

Many liquids, such as alcohol, ether, and bisulphide of carbon, do not solidify even at the lowest known temperature. Pure water solidifies at zero; salt water at -2.5°, olive and rape oils at −6°; linseed and nut oils at -27°.

Water presents the remarkable phenomenon, that when it solidifies and forms ice its volume undergoes a material increase. In speaking of the maximum density of water we have already seen that, on cooling, it expands from 4 degrees to zero; it further expands on the moment of solidifying, or contracts on melting, by about 10 per cent. One volume of ice at o° gives 0908 of water at o°, or I volume of water at o° gives 1.102 of ice at the same temperature.

The increase of volume in the formation of ice is accompanied by an expansive force which sometimes produces powerful mechanical effects, of which the bursting of water pipes and the breaking of jugs containing water are familiar examples. The splitting of stones, rocks, and the swelling up of moist ground during frost, are caused by the fact that water penetrates into the pores and there becomes frozen.

The expansive force of ice was strikingly shown by some experi

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