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to convert Regnault's table into one expressing the pressure of vapour at any temperature in inches of mercury at o°C at the place of observation, but if we use degrees Fahrenheit we shall find that Regnault's tables do not give us the pressure at our precise points of temperature, and we shall have to calculate these as Regnault himself did, by adapting his formulæ of interpolation to our case. In this adaptation it must be borne in mind that 212° Fahr. is not exactly 100°C.

The Rev. Robert Dixon, in his work on Heat, has made the requisite adaptation with great care, and we cannot do better than present to our readers in an abbreviated form the tables he has calculated.

They are adapted to the latitude 53° 21', which will answer very well for any station in Great Britain or Ireland. These tables, along with an abbreviation of Regnault's table, will be found at the end of this work; they compriseTable I. Shewing the elastic force of aqueous vapour in

inches of mercury at the latitude 53° 21′

for each degree Fahr. from -30° to 432°. Table II. Shewing the elastic force of aqueous vapour in inches of mercury at the same latitude

from o° to 100° Fahr. for every two-tenths of a degree.

Table III. Shewing the elastic force of aqueous vapour in millimètres of mercury at the latitude

of Paris (48° 50') for each degree Centigrade from 32°C to + 230°C.

144. Pressure of other vapours. Signor Avagadro and M. Regnault have both determined the tension of the vapour of mercury at various temperatures, but their results can only be considered an approximation. Regnault has also determined the tension of other vapours at various temperatures, and the following table gives the results of some of his experiments.

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145. The change of condition from liquid to solid is without influence on vapour tension. From experiments made by Gay Lussac and Regnault it would appear that the passage of a substance from the liquid to the solid state is without influence upon the vapour densities, so that in the curve which embodies M. Regnault's observations on the elasticity of aqueous vapour there is no break at the freezing point.

146. Dalton's hypothesis regarding vapour tensions. Dalton supposed that the tensions of all vapours would be equal at equal distances from their respective boiling points. Thus, for example, water boils at 212° Fahr. and alcohol at 173°. Of course at these temperatures the tension of both vapours is equal, being represented by one atmosphere, or 30 inches of mercury. According to Dalton the tensions of these two vapours will also be equal at

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This hypothesis of Dalton does not generally hold, but for short distances on either side of the boiling point it holds approximately in a large number of instances.

When sub

147. Density of gases and vapours. stances are compared together in the state of gas at the same temperature and pressure, a very simple relation is found to subsist between their density and their combining chemical equivalent. This was first discovered by Gay Lussac, who found that when gas or vapours combine together, the volumes in which they combine bear a very simple ratio to one another. The following table, kindly furnished by Dr. Williamson, will serve to illustrate this law, which is generally called the law of volumes.

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The new notation is used.

Vapour volume of elements libe-
rated from them.

I vol. hydrogen + I vol. chlorine.
I vol. hydrogen + 1 vol. bromine.
I vol. hydrogen + 1 vol. iodine.
2 vols. hydrogen + I vol. oxygen.
I vol. nitrogen +3 vos. hydrogen.

We see from this table that equal volumes of chlorine and hydrogen, for instance, combine together without change of volume to form hydrochloric acid gas, which contains one atom of hydrogen united to one of chlorine. Equal volumes of chlorine and hydrogen contain therefore an equal number of atoms of these elements.

There are thus three laws which bear immediately upon the density of gases. 1. The above law of volumes, in which the density of a gas at a given temperature and pressure is shewn to depend upon its chemical constitution; 2. Boyle's law, in which the density of a gas of a given constitution and temperature is shewn to depend upon its pressure; 3. Gay Lussac's law, in which the density of a gas of a given constitution and at a given pressure is shewn to depend upon its temperature.

148. Many experiments have been made with a view to determining whether these three laws hold for all gases and vapours, and we shall now very briefly indicate the various methods pursued in these investigations.

In the first place, Gay Lussac's method in his researches consisted in ascertaining the volume occupied by a known weight of liquid when entirely converted into vapour at a certain temperature and pressure.

It is, however, essential to this method that the whole of the liquid should be converted into vapour, and hence it is inapplicable to a gas at its maximum density and in contact with its own liquid. It only applies to these cases when the density is considerably inferior to that of saturation. No doubt the density of saturation might be calculated from an experiment of this kind, if we supposed Boyle's law to hold good; but one great object of such experiments is to determine whether this law holds accurately for gases near their point of saturation.

In order to obviate this objection M. Despretz introduced a method which consists in filling with gas or vapour at different temperatures and pressures a balloon of known weight screwed on the top of a barometer tube.

Afterwards Dumas, in order to experiment upon gases that act upon mercury, and to obtain results at high temperatures, used a glass balloon, which he arranged so as to

be filled with the vapour of a liquid at a temperature 20° or 30° above the boiling point of the liquid, and under the ordinary atmospheric pressure.

More lately Regnault has made several series of experiments on this subject.

1. His first series was on the density of aqueous vapour in vacuo at the temperature of boiling water, and under a pressure not exceeding half an atmosphere, and he found that both Boyle's and Gay Lussac's laws were applicable within the limits of his experiments, but when the pressure approached more nearly to its maximum, he found that the density increased more rapidly than the elastic force.

2. His second series was on the density of aqueous vapour in vacuo at temperatures not very far removed from that of the surrounding medium, and from these he concludes that the density of aqueous vapours in vacuo and under feeble pressures may be calculated according to Boyle's law, provided that the fraction of saturation does not exceed o.8, but that this density is notably greater when we approach more nearly to the state of saturation. He adds, however, that this latter circumstance may be owing to one or both of two causes; either aqueous vapour really suffers an anomalous condensation on approaching its point of saturation, or a portion of the vapour remains condensed on the surface of the glass, and does not assume the aëriform state until the mass of vapour is at some distance from the point of saturation.

3. Regnault has also examined the density of aqueous vapour in air at its maximum value for the temperature of experiment between the limits o° and 25°C, and he concludes that the density of aqueous vapour in air, in a state of saturation and under feeble pressures, may be calculated, without much error, from Boyle's law.

Messrs. Fairbairn and Tate have lately made experi

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