The effect of a continued addition of heat to a liquid is, as we have remarked above, to convert the liquid into the state of vapour. This, like the former, is a change, and, like it, is attended with a complete alteration in the physical properties of the substance so changed. A certain portion of caloric added to a body, insinuates itself among the particles, and places them so far from each other, as to diminish the attraction which unites them into the so lid form, without, however, entirely destroying their ag gregation ; while a larger portion removes them to a still distance, and seems as if it entirely placed them beyond the reach of their mutual attraction. The most ob vious property of the body, when in this vaporized state, is its power of expanding itself in an indefinite degree, until its particles be brought nearer together by an exter nal compressing force. If this external force be remov ed, it quickly regains its former dimensions ; and hence we regard it as a body possessed of a perfect degree of elas ticity. It is upon the elastic power which different substan ces possess, when they are converted by heat into the gase ous or acriform state, that many of the most important operations, both in art and nature, are brought about. The force which different bodies exercise when they are reduced into the gaseous state, and are at the same time subjected to strong pressure, is almost inconceivable ; it is, in short, unlimited in its extent, and may be so managed as to pro duce any assignable degree of effect. Notwithstanding, however, the expansive power which bodies exercise dur ing their conversion from the fluid to the gaseous form ; yet when they are finally brought into this state, they are compressible. Thus, although' the conversion of water into an aeriform fluid is capable of generating an almost immense degree of mechanical power, in consequence of the greater space which it then occupies, a power of which we take advantage in the steam engine, yet the vapour, when once produced, may be easily compressed into a less bulk, by the weight of a comparatively small quantity of mercury. Farther increments of heat, when added to the gas, only serve to produce a corresponding increase of its bulk ; and this increase, as we have already remarked, is equable for the same quantity of heat, and is likewise simi lar in all the different gaseous bodies.
The point of vaporization, like that of solidification, differs for almost every kind of liquid. There are some bodies that always appear in the state of gas when not combined with other substances, and no degree of cold which has yet been applied to them has been able to deprive them of their elastic fluidity. Yet from analogy we conclude, that these bodies have nothing in their nature essentially different from those which we observe to assume either the liquid or the gaseous state, according to the temperature in which they are placed ; and we conceive, that, had we the power of employing a degree of cold sufficiently intense, they might be rendered liquid, or solid. Ether has the lowest and mercury the highest vapour points of any liquids with which we are acquainted. If we were always im mersed in a temperature above that at which ether boils, we should never see it in a fluid state ; and if we had not been able to produce a temperature above 670°, at which mercury boils, we should have had no conception of this substance being reducible into the state of a vapour. The
terms fixed and volatile, like solid and liquid, are therefore to be regarded as merely relative, and as not indicating the absolute quality of the bodies, but pointing out the forms under which they usually present themselves to our notice. But although we regard this as the most correct view of the subject, yet many chemists of eminence have suppos ed that there is some essential or radical difference between the two kinds of aeriform fluids, and have called them by different names ; to those that are easily reduced to the liquid state, the term vapour has been exclusively applied, while the others have been called gases, or permanently elastic fluids. The discussion of this question belongs more to chemistry, than to the immediate subject of this article; but we shall farther remark concerning it, that when, by the effect of chemical affinity, these gases are reduced to the liquid or solid form, heat is extricated ; thus proving that it entered into their constitution, and was necessary to their existence.
An important circumstance connected with the conver sion of liquids into the state of elastic fluidity, is, that it is influenced by the degree of pressure to which the li quid is subjected. External pressure thus counteracts the repulsive power of heat, and as it were keeps the par ticles in a state of forcible approximation. It follows, from this order of things, that if we remove from the surface of a liquid the whole or a part of the weight of the atmos phere, by placing it under the receiver of an air-pump, or by ascending a high mountain, it boils at a lower tempe rature than under ordinary circumstances. Thus water, which boils at 212° when the barometer stands at 30 inch es, boils at about 70° in vacuo; and on the top or Mount Blanc, it was found by Saussure to boil at about 187°. As the temperature at which different species of elastic fluids are formed depends so much upon the pressure which is applied to them ; so, after the gaseous state has been assumed, the elastic force of the vapour varies accord ding to the degree of pressure under which it exists. Mr Dalton has made this the subject of an extensive range of experiments, in which he determines the height of the the mercurial column which the vapour of water will sup poi t at different temperatures. Thus if the vapour of water be taken at the freezing and boiling points, and at which is intermediate between them, he found that the column of mercury supported in these three cases was equal to 0.2, 3.5, and 30 inches respectively. By making a few experiments, and observing the ratio which the numbers in these cases bore to each other, he constructed a table containing every degree of heat, from 0° on Fah renheit's scale to 325° ; (Manch. Men:. vol. v. p. 559.) Experiments of a similar kind have been performed in France by M. Bettancour. As the boiling point of liquids is lowered or raised, according as the atmospherical pres sure upon them is diminished or increased, it has been found, that if the pressure be augmented in a very great de gree, as is the case when water is inclosed in Papin's di gester, it is capable of receiving a much higher tempe perature than what it can maintain under other circUmr stances. Water has, in this way, been heated 200° or 300° beyond its usual boiling point ; and it has then acquir red the power of dissolving many substances, over which it has no action in its ordinary state.