ments to determine the density of steam at different temperatures, and to find the law of expansion of superheated

steam.

The general plan of their method of ascertaining the density of steam consists in vaporizing a known weight of water in a large glass globe with a stem, of known capacity and devoid of air, and observing the exact temperature at which the whole of the water is just vaporized.

In the following table the authors exhibit the relation between the specific volume, pressure, and temperature of saturated steam as determined from their experiments. Specific volume denotes the number of times the volume of steam exceeds the volume åt 39°.1 Fahr. of the water from which it is raised.

With regard, in the next place, to superheated steam, the results of these experiments shew that for temperatures within about ten degrees from the maximum temperature of saturation, the rate of expansion on account of heat greatly exceeds that of air, whereas at higher temperatures from this point the rate of expansion approaches that of air, so that as the steam becomes more and more superheated, the coefficient of expansion approaches that of a perfect gas, while at or near the maximum temperature of saturation the coefficient of expansion greatly exceeds that of a perfect gas.

We thus perceive that near their points of saturation gases and vapours would appear to depart both from Boyle's and from Gay Lussac's law, while probably if the pressure under which they exist be far inferior to that of saturation these laws are obeyed.

Generally speaking we may presume that the three laws to which we have before alluded as regulating gaseous density (namely the law of volumes, Boyle's law, and Gay Lussac's law), only hold accurately in the case of perfect gases.

149. Regnault has furnished us with the following determination of the weight of a litre of the most important gases.

Weight of one litre (61.02705 cubic inches) of air, oxygen, hydrogen, nitrogen, and carbonic acid gas.

150.

Hygrometry.

Hygrometry is that branch of

science which treats of the state of the air with regard to moisture. As this is one of the elements which form the climate of a place, and as the human body is very much affected by the hygrometric state of the air, the subject is one of much practical importance.

There are several facts regarding the vapour present in the air which it is very desirable to know.

151. One of these is its Tension. Suppose that we were to isolate in a vessel a cubic foot of air, allowing it to remain at its present temperature and pressure, and then to introduce into the vessel containing it a substance which absorbs moisture; the air by this means will be rendered dry, and its tension will be diminished by an amount representing the tension of aqueous vapour present in

the air.

It is of importance to know what this tension is, for upon this, among other things, depends the behaviour of the air when it is cooled down. If, for instance, at the higher temperature there be present nearly as much aqueous vapour as the air can contain at that temperature, then if the air be cooled down only a few degrees, some of this vapour will be deposited in the liquid or solid state. The temperature at which this takes place is called the dew-point. We thus see that if the tension of vapour in the air at its existing temperature be great the dew-point will be high, but if this tension be small, the dew-point will be low in the thermometric scale.

152. Another object of research is the relative humidity

of the air. Of course all substances exposed to the air will be affected by the deposition of moisture when the dew-point is reached, but many substances will be affected long before this takes place; our bodies, for instance, will experience the wetness of the air long before. On the other hand, if the present temperature be far above that of deposition, we pronounce the air dry. It ought here to be observed that the sensation of dryness or wetness does not depend upon the absolute amount of aqueous vapour present in one cubic foot of air. For if the temperature be very low, although the air may not contain much aqueous vapour, yet this vapour may approach very nearly to the maximum amount which can be retained at the temperature, and the air will be pronounced wet. But if the very same mixture of air and vapour be heated up many degrees, the vapour will represent only a small fraction of the total amount which can be retained at the higher temperature, and hence it will feel very dry. If this high temperature be produced by a stove, it may even be necessary to place near the stove a vessel containing water in order to increase the amount of aqueous vapour present in the air.

We see now what is meant by the dryness or wetness of the air, and all that remains is to express it numerically. This is done by the conception of relative humidity, which may be thus defined. Relative humidity is the fraction expressing the ratio between the tension of vapour actually present in the air at a given temperature and the greatest amount of vapour which it can contain at that temperature. The greatest amount, representing complete saturation, is generally reckoned equal to 100, and on this principle 50, 40, 30, &c. will denote that the air contains 50, 40, 30, &c. per cent. of the maximum amount which can be contained at that temperature.

153. The weight of vapour present is another object of

interest. In order to know completely the state of the air, it is necessary to know the weight of vapour present in a given volume of air, and also the entire weight of a given volume of air, or its specific gravity. This last element is necessary on another account, for a body weighed in air is lighter than if weighed in vacuo by the weight of its own bulk of air; in very delicate weighings, therefore, it is necessary to find the exact weight of the air displaced by the body; and in order to obtain this information it is not sufficient to know the temperature and pressure of the air, but we must also know the weight of vapour contained in a given volume of air.

154. Having now mentioned the objects sought in hygrometry, let us proceed to describe shortly the various instruments made use of in this science. We may state at the commencement that there are various means of ascertaining in a general way the dryness or wetness of the air. We may, for instance, use some substance which has a great affinity for water and readily deliquesces. Such a substance, if the air be very dry, will remain a long time comparatively unaffected, but if the air be moist it will rapidly deliquesce. In the next place, various substances have the property of becoming elongated when moist and of contracting again when dry; a hair, for instance, possesses this property, and Saussure has used it in his hair hygroscope. Other bodies, such as catgut, untwist when moist and twist when dry; and a toy has been made in which there are two figures, a man and a woman, suspended by catgut in such a manner that the man comes out when the air is wet and the woman when it is dry. All these methods, however, indicate rather than measure the hygrometric state of the air-they are hygroscopes rather than hygrometers—and we proceed from these to instruments by which the state of the air with regard to moisture may be determined with precision.