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feature of this phenomenon. There is probably, however, a layer of vapour between the two surfaces.

131. Vaporization of liquids at a very high tem

perature and in a limited space. When a liquid is enclosed within a limited space, and heat is applied, each increment of heat produces a certain quantity of vapour, which thus accumulates, and by its pressure prevents the liquid from boiling, the temperature of the liquid meanwhile rising considerably above its ordinary boiling point. M. Cagniard de la Tour pushed his observation of this process to an extreme limit by enclosing certain liquids in a space not much larger than their own volume. On the application of heat he found that the liquids at a certain temperature passed completely and instantaneously into the state of vapour. The apparatus which he employed for this experiment had within it some atmospheric air shut out from the liquid by a drop of mercury. As the liquid became heated its vapour pushed the mercury before it along a tube, compressing, by this means, the air into a very small volume, Thus, if the volume finally occupied by the air was onefiftieth of that which it would have filled under the ordinary pressure, it might be concluded that the pressure was equal to fifty atmospheres, and so on. M. Cagniard de la Tour made experiments on ether, alcohol, bisulphide of carbon, and water, and obtained the results given in the following table.

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There was considerable difficulty in performing the experi

ment with water, as at the high temperature to which it was raised its power of dissolving glass was very great.

It thus appears that at a certain temperature a liquid under the pressure of its own vapour becomes changed into a gas. Faraday has imagined that at this temperature, or one a little higher, no pressure that we are likely to produce would convert the gas into a liquid, and he also thinks that the temperature of 166° Fahr. below zero is probably above this limiting point for the gases oxygen, hydrogen, and nitrogen; so that, apply what pressure we may, we shall not be able to liquefy these gases unless we can at the same time produce intense cold.

Dr. Andrews has recently noticed that when a vessel containing liquid carbonic acid is raised to 88° Fahr. the surface of demarcation between the liquid and gas becomes gradually fainter, loses its curvature, and at last disappears *. The space is then occupied by a homogeneous fluid, which exhibits, when the pressure is suddenly diminished or the temperature slightly lowered, an appearance of moving or flickering striæ throughout its entire mass. At temperatures above 88° no apparent liquefaction of carbonic acid could be effected even under the pressure of 300 or 400 atmospheres. He obtained similar results with nitrous oxide.

If we consider capillary curvature to be one of the characteristics of a liquid, the gradual diminution of curvature in the foregoing experiment would seem to imply a gradual transition from the liquid to the gaseous state, and to lead to the belief that there may be an intermediate condition of matter between the liquid and the gaseous, just as we have in a viscous fluid a link between a solid and a liquid.

132. Sublimation. There are some solids which under ordinary circumstances appear to assume the gaseous state at once instead of passing through the intermediate state of liquidity. Of these arsenic and solid carbonic acid are

* A similar change of curvature has been noticed for other liquids by Wolf, and also by Drion.

examples. It is well known too that snow slowly evaporates, and thus to some extent appears to assume the gaseous form, even although the temperature is decidedly below that of the melting point of ice.

Again, some substances, such as chalk, are decomposed before fusion, the gaseous element going off. Nevertheless it has been found that if chalk be heated under intense pressure it melts and assumes the appearance of marble when it becomes solid.

CONDENSATION OF VAPOURS AND GASES.

133. Distillation, or the condensation of vapours. This process has already been described in Art. 117.

134. Condensation of gases. Sometimes gases are compelled to assume the liquid form by their affinity for some liquid. Thus, for instance, if ammoniacal gas is brought into contact with water it is immediately dissolved. The same result takes place with hydrochloric acid and other gases. This condensation of gas renders sensible a large quantity of latent heat, and it is therefore necessary to keep the vessel in which it takes place surrounded by cold water.

It is much more difficult to condense gases by themselves and without the aid of a chemical solvent. The auxiliaries employed in this condensation are, as might be imagined, pressure and cold. A number of gases have yielded to the joint effect of these two agents, but there are nevertheless six substances which we have not yet been able to obtain either in the liquid or the solid form—these are oxygen, hydrogen, nitrogen, nitric oxide, carbonic oxide, and marsh gas.

Faraday was one of the first who succeeded in liquefying gases by the joint application of cold and pressure. Others have since joined in these attempts, and carbonic acid gas is now condensed in large quantities, forming in this

An instrument

shape a very convenient source of cold. invented by Thilorier, somewhat modified, is very much used for the purpose of liquefying this gas-it acts on the following principle:-The gas is generated in a strong iron vessel called the generator, into which the materials necessary for making the gas are put. This generator is connected with an equally strong iron vessel called the receiver, which is kept cool, and when sufficient gas has been generated it condenses in the receiver. The two vessels are now separated and a fresh charge introduced into the generator, and the gas is condensed in the receiver as before, until at last a large quantity of liquefied gas has been obtained.

Fig. 27.

It will be seen from Fig. 27 that in the interior of the receiver there is a tube which descends below the level of the liquid. When the cock is opened the pressure of the gas drives the liquid with great force up the tube and out through the fine nozzle in which it terminates. The liquid as it issues evaporates with such rapidity that part of it is frozen, and if on its way out it be made to play into a cylindrical box the solidified gas may be collected in the form of a snow-white powder. This powder evaporates very slowly, and may therefore by proper precautions be preserved for a considerable length of time. It may also be handled with impunity, and may even be laid upon the tongue without a disagreeable sensation of cold being produced, although the temperature of the solid is extremely low, perhaps even-106° Fahr. The reason of this absence of the feeling of cold is want of contact between the two. If however this solid acid be mixed with ether, for which it has a great attraction, the mixture will now

be in contact with the containing vessel, and will be felt to be intensely cold, while a rapid evaporation of carbonic acid will take place, thereby preserving the low temperature by drawing off the heat.

By means of a mixture of this description from 20 to 30 pounds of mercury may readily be frozen. If the bath of carbonic acid and ether be placed in vacuo the evaporation is accelerated, and a still greater degree of cold is produced. Faraday by this means has reached the temperature Fahr. A still lower temperature has been obtained by Natterer, who by means of a bath of liquid nitrous oxide and bisulphide of carbon in vacuo has reached the temperature -220° Fahr.

- 166°

ELASTIC FORCE AND DENSITY OF VAPOURS AND GASES.

135. Before discussing the laws which regulate the elastic force of gases and vapours we must first of all bear in mind what takes place when we condense a gas or vapour into smaller volume. Let us suppose this condensation to be performed slowly and at a constant temperature. As it proceeds the pressure will of course increase, until at last, if the gas be condensable, liquid will begin to make its appearance. The pressure and density have now attained the greatest possible value which they can have for this temperature and for this kind of gas, and if the condensation be pushed further the only result will be the formation of more liquid, but without a further increase of pressure. Our inquiry must therefore be divided into two parts. We must consider, in the first place, the laws which regulate the pressure of gas or vapour not in contact with the liquid which produces it; and, secondly, those which regulate the pressure of gas or vapour in contact with its own liquid.

136. Pressure of gas or vapour not in contact with its own liquid. We have already had occasion (Chap. IV. Art. 60) to advert to Boyle's law, which gives the relation

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