The idea of employing very powerful pressures, acting through a short space, seems more valuable at first sight than it proves on examination. It is considered that an engine of high power can be got into a small place, and will be of less weight. But the real inconveniences, are, the large mass of fuel required to supply the engine a given time, and the immense surface that must be exposed to an intense heat to obtain a given quantity of heat in a given time. Besides, when we attempt to use high degrees of pressure, an accuracy of workmanship, and attention to the elasticity of materials, becomes necessary, which renders the work expensive, and of short duration.

The success of Mr. Faraday in reducing various gases into the liquid state is not however the less important. His method consisted in generating the substances in a bent tube of glass hermetically sealed at both ends. Then, by cooling one end of the bent tube and heating the other, when heat was necessary, the gas was condensed in a liquid state at the cold end of the tube.

117.-Carbonic acid required the greatest precautions to effect the condensation with safety. The liquid obtained is a limpid, colourless body, extremely fluid, and floated upon the contents of the tube, without mixing. It distils readily at the difference of temperature between 32° and 0°; its refractive power is much less than that of water, and its vapour exerts a pressure of thirty-six atmospheres at a temperature of 32o. In endeavouring to open the tubes which contained it, at one end, Mr. Faraday states, that they uniformly burst with powerful explosions.*

The gases reduced to a liquid state by Mr. Faraday, with their densities as far as known, are collected in the following table, with a column to shew the mechanical power compared with steam.†

* The ingenious Mr. Brunel is attempting to work an engine where the acting vapour is to be liquid carbonic acid. It is to be regretted that his great talent for mechanical combination should be employed where there is so little chance of success.

+ The power is as the force and the space through which the gas passes in its reduction to the state of liquid, (See Sect. IV.) The space is found by comparing the density of the body in the liquid state with its density in the gaseous under the same pressure; and as the weight of air is to water as 1 : 828; to find the mechanical power of equal volumes of the liquid, we have simply to multiply 828 by the specific gravity of the liquid, and divide the product by the specific gravity of the body in the state of gas. The force does not enter into the calculation, because the density of the gas must obviously be greater in the same proportion. The quantity of heat is most probably in the ratio of the power, and if this be the case, all substances will afford equal powers with equal quantities of heat.

These are the principal researches that have been made on the force of vapours at different temperatures, when in contact with liquids; but in order to render the subject more complete, we must consider the force when not in contact with the liquids which generate them, and their density and volume.

Of the elastic Force of Vapour separated from the Liquids from which they
were generated.

118.-It has been remarked, that the elastic force of steam or vapour produced by increase of temperature, ceases to follow the same law where it is not in contact with the liquid from which it was formed, (art. 87.) The density of the steam no longer increases, the force being solely that which prevents it expanding, and is measured from the quantity it would expand if unconfined. The expansion by the same increase of temperature having been found to be the same in all gases and vapours, and the density as the compressing force, as far at least as 60 atmospheres, it becomes an easy task to compute this species of force within that range of compressive force.

This will also be further useful in determining the volume steam of a given density and temperature occupies as far as about 60 atmospheres; higher we need not attempt to go for useful purposes; and if we did our rules would fail, for there is not even a probable

chance of the law of the density being as the force extending to very high degrees of compression.

119. The quantity a gas or vapour expands is found by the following rule. RULE. To the temperature before and after expansion, add 459. Then divide the greater sum by the less, and the quotient multiplied by the volume at the lower temperature, will give the volume at the higher temperature.

Or let t be the temperature with the volume v, and t any other temperature, then

As the volume the vapour occupies at the lower temperature is to the volume it would become by expansion, so is the elastic force at the lower temperature to that at the higher

Taking as an example M. Cagniard de la Tour's experiments on ether, it is stated, that it was completely in a state of vapour at a lower degree, but the differences do not indicate this to have taken place till it was 447°, and its force was 688 atmospheres; required its force at 617.° In this case

In the experiment he states it as 94 atmospheres, and undoubtedly in consequence of the vapour of mercury forming in the apparatus, (art. 107.) and a like remark applies to all his experiments; for our rule for the expansion rather exceeds the truth than otherwise.

120. By reversing the process, we may find the volume steam will occupy under any compressive force not exceeding 60 atmospheres, when its volume is known for a given temperature and pressure. For example at 60° its force being thirty inches of mercury, its volume is 1324 times its volume in water.* Now by increasing its temperature

The volume of any vapour or gas at 60° and 30 in. is easily found from chemical tables containing their specific gravity, compared with air at that temperature and pressure; for air is 828 times the volume of an equal weight of water: consequently, the number 828 being multiplied by the specific gravity of the liquid, and divided by the specific gravity of the vapour in question, gives its proportion of volume to an unit of volume of the

= 2·53 (459 + 1′).

30 × 2·55 (489 + ť)
f

cuperature ť.

76·5 (459 + 1')

=

A convenient rule for finding the volume or space the steam cups, when the steam is of any given elastic force and tem

The temperature in degrees, and multiply the sum by 765. yae force of the steam in inches of mercury, and the result will be ea of a cubic foot of water will occupy.

aerocce of the steam be four atmospheres, or 120 inches of mercury, the force being according to Mr. Southern's experiments 295°, (art. 77.) 70-4; and

ved wound by experiment was 404; and considering the difficulty of ascertain

From this table it appears that one volume of water produces more vapour than an equal volume of any other substance in the list.

ing the volume, on account of the allowances to be made for escape of steam of such a high temperature, it agrees very well with the calculated result. According to Dr. Ure's experiments the force of steam at 295°, is 129 inches, which gives 446 for the times the volume is increased by converting into steam of that force and pressure.

Of the Mixture of Air and Steam.

122.-It is a well known fact that common water contains a considerable portion of air or other uncondensible gaseous matter, and when water is converted into steam, this air mixes with it, and when the steam is condensed remains in the gaseous state. If means were not taken to remove this gaseous matter from the condenser of an engine, it would collect so as to obstruct the motion of the piston. But even when means for removing it are employed, a certain quantity constantly remains in the condenser of an engine, and in order to determine its state, we must consider the effects produced by mixing air with steam, or vapour, at different temperatures and pressures.

Let us suppose that we have air and vapour of the same temperature t, and elastic force p; and that the volumes are v and v'. If they were now put one on the other in a closed vessel of the capacity v + v', it is plain they could preserve an equilibrium; because, the temperature is the same, and the mutual pressures are equal; but this equilibrium would not be stable.

Experience proves that these gases would gradually mix together till they became completely intermixed. It further shews that during this operation heat is neither evolved nor absorbed; so that after a certain time the mixture is perfectly homogeneous, the two gases holding the same proportion in every part, and the temperature and pressure being t and p. From these facts, established by observation, we may deduce another equally well verified by experience.

123.-If two gases, or a gas and vapour, mixed together at the temperature t fill a volume v; and if p and ƒ denote the pressures they would separately exert when separately occupying the same volume v, at the same temperature t, the pressure of the mixture will be p + f.

In effect, let us suppose that the two gases at first are distinct, and let ƒ be greater than p; then dilating the gas under the pressure f, until ƒ changes to p, its volume will be

come

v f
P

N