improved by Reboul*, and by Seguin and Lavoisier +; but Berthollet has lately brought it to a state of perfection.

Instead of the rapid combustion of phosphorus, this last philo sopher has substituted its spontaneous combustion, which absorbs the oxygen of air completely; and when the quantity of air ope rated on is small, the process is over in a short time. The whole apparatus consists in a narrow graduated tube of glass containing the air to be examined, into which is introduced a cylinder of phosphorus fixed upon a glass rod, while the tube stands inverted over water. The phosphorus should be so long as to traverse nearly the whole of the air. Immediately white vapours rise from the phosphorus and fill the tube. These continue till the whole of the oxygen combines with phosphorus. They consist of phos phorous acid, which falls by its weight to the bottom of the vessel, and is absorbed by the water. The residuum is merely the azotic gas of the air, holding a portion of phosphorus in solution. Berthollet has ascertained, that by this foreign body its bulk is increased 1-40th part. Consequently the bulk of the residuum, diminished by 1-40th, gives us the bulk of the azotic gas of the air examined; which bulk, substracted from the original mass of air, gives us the proportion of oxygen gas contained in it §.

All the different experiments which have been made by means of this eudiometer agree precisely in their result, and indicate that the proportions of the ingredients of air are always the same; namely, about 0·21 parts of oxygen gas, and 0.79 of azotic gas. Berthollet found these proportions in Egypt and in France, and they have been found constantly in Edinburgh, in all the different seasons of the year.

Thus it appears, that whatever method is employed to abstract oxygen from air, the result is uniform, provided the experiment may be precisely made. They all indicate that common air consists very nearly of 21 parts of oxygen and 79 of azote. and Lavoisier found 27 per cent. of oxygen, but their methods

Ann. de Chim. xiii. 38.

+ Ann. de Chim. ix. 293.

Ibid. xxxiv. 73. and Jour. de l'Ecole Polytechn. I. iii. 274.

Scheele

A very convenient apparatus for making eudiometrical experiments, has lately been invented by Mr. Pepys, and described by him in Phil. Trans, for 1807.

were not susceptible of precision. Air, then, does not vary in its composition; the proportion between its constituents is constant in all places and in all heights. Gay Lussac examined air brought from the height of more than 21,000 feet above Paris, and found it precisely the same as the air at the earth's surface*.

But 21 cubic inches of oxygen gas weigh 9-14 grains, and 79 inches of azote weigh 23-9686 grains. These added together amount to 31.1086 grains, which ought to be the weight of 100 inches of common air. But this is somewhat greater than the weight of 100 inches of air, according to Sir John Shuckburgh' Evelyn's experiments, who found it only 31-0197 grains. The difference is not great, and is probably owing to a small error in the specific gravities of the different gases. According to this es timate, 100 parts of air are composed by weight of

22.91 oxygen

77.09 azote

100.000

[Mr.Dalton considers air as merely a mechanical mixture of the two gases of which it is compounded. But all other chemists consider it as a chemical compound: most of them as a compound of azote and oxygen holding each other in dissolution.

[Thomson's Chemistry.]

SECTION 111.

Asmospheric Water.

THAT the atmosphere contains water has been always known. The rain and dew which so often precipitate from it, the clouds and fogs with which it is often obscured, and which deposite moisture on all bodies exposed to them, have demonstrated its existence in every age. Even when the atmosphere is perfectly transparent, water may be extracted from it in abundance by certain substances. Thus, if concentrated sulphuric acid be exposed to air, it gradually attracts so much moisture, that its weight is increased more than three times it is converted into diluted acid, from which the water may be separated by distillation. Substances

Phil. Mag. xxi. 225.

which have the property of abstracting water from the atmosphere have received the epithet of hygroscopic, because they point out the presence of that water. Sulphuric acid, the fixed alkalies, muriate of lime, nitrate of lime, and in general all deliquescent salts, possess this property. The greater number of animal and vegetable bodies likewise possess it. Many of them take water from moist air, but give it out again to the air when dry. These bodies augment in bulk when they receive moisture, and diminish again when they part with it. Hence some of them have been employed as hygrometers, or measures of the quantity of moisture contained in the air around them. This they do by means of the increase or diminution of their length, occasioned by the addition or abstraction of moisture. This change of length is precisely marked by means of an index. The most ingenious and accurate hygrometers are those of Saussure and Deluc. In the first, the substance employed to mark the moisture is a human hair, which by its contractions and dilatations is made to turn round an index. In the second, instead of a hair, a very fine thin slip of whalebone is employed. The scale is divided into 100°. The beginning of the scale indicates extreme dryness, the end of it' indicates extreme moisture. It is graduated by placing it first in air made as dry as possible by means of salts, and afterwards in air saturated with moisture. This gives the extremes of the scale, and the interval between them is divided into 100 equal parts.

Since it cannot be doubted that the atmosphere always contains water, there are only two points which remain to be investigated: 1. The state in which that water exists in air; 2. The quantity which a given bulk contains.

I. With respect to the state in which water exists in air, two opinions have been formed, each of which has been supported by very able philosophers. 1. Water may be dissolved in air in the same manner as a salt is held in solution by water. 2. It may be mixed with air in the state of steam or vapour, after having been converted into vapour.

1. The first of these opinions was hinted at by Dr. Hooke in his Micrographia, and afterwards proposed by Dr. Halley; but it was much more fully developed by Mr. Le Roy of Montpelier in 1751. Dr. Hamilton of Dublin made known the same theory about the same time. The phænomena in general coincide remarkably well with this theory. The quantity of water which

air is capable of holding in solution is increased by every augmen. ation of temperature,' and diminished by cold, which is precisely analogous to almost all other solvents. These analogies, and several others which will easily suggest themselves to the reader, have induced by far the greater number of philosophers to adopt this opinion.

2. The second theory, namely, that water exists in air in the state of vapour, has been embraced by Deluc in his last treatise on Meteorology; at least his reasoning appears to me to lead to that conclusion. But it is to. Mr. Dalton that we are indebted for the most precise information on the subject. The following reasons put the truth of this opinion almost beyond the reach of controversy.

In the first place, It cannot be doubted that the water which exists in air, is derived originally from the waters on the surface of the earth, which are exposed to the action of the atmosphere. Accordingly we find that water, when exposed to the air, suffers a gradual diminution of bulk, and at last disappears altogether. This diminution of the water may be owing, either to its gradual solution in air, or to its convertion into vapour. The last is the common opinion, as the phenomenon is in common language ascribed to the evaporation of the water. When water is placed in an exhausted receiver, it diminishes in bulk even more rapidly than in the open air. In this case, as no air is present, we can only ascribe the diminution of bulk to the conversion of the water into vapour. Accordingly we find, upon examination, that the receiver is actually filled with water in the state of vapour. The presence of this vapour very soon, by its elasticity, puts an end to the evaporation of the water. Now, since water disappears equally whether air be present or not, and exactly in the same manner, it is reasonable to ascribe its disappearing in both cases to the same cause. But in the exhausted receiver it is converted into vapour. Hence it is probable that it is converted into vapour also in the open air; and if so, it must exist in air in the state of vapour.

In the second place, if the disappearing of water exposed to the open air were owing to solution and not to evaporation, it ought certainly to disappear more rapidly when it is exposed to the action of a great quantity of air then when to a small quantity; for the

• Manchester Memoirs, v. p. 517.

quantity of any body dissolved is always proportional to the quan tity of the solvent. But the very contrary is what actually takes place with respect to the water contained in the air. Saussure has proved that water evaporates much faster at great heights than at the surface of the earth, even when the temperature and the moisture of the air in both places are the same. By comparing a set of experiments made upon the Col-du-Geant, at the height of 11,275 feet above the level of the sea, with a similar set made at Geneva, 1324 feet above the level of the sea, he ascertained, that supposing the temperature and the dryness of the air in both places the same, the quantity of water evaporated at Geneva is to, that evaporated on the Col-du-Geant in the same time and same circumstances as 37 to 84, or nearly as 3:7. Now the air on the Col-du-Geant is about one-third rarer then at Geneva; so that the diminution of about one-third in the density of the air more than double the rate of evaporation*. This is precisely what ought to be the case, provided the water which disappears mixes with the air in the state of vapour only; but the very contrary ought to hold, if the water disappeared in consequence of the solvent power of air.

In the third place, it has been demonstrated by Dr. Black, that vapour is water combined with a certain dose of caloric. Conse quently when water is converted into vapour, a certain portion of caloric combines with it and disappears. If, therefore, there is the same waste of caloric whenever water passes from a liquid state, and enters into the atmosphere as a component part, we have reason to conclude that it enters into the atmosphere only in the state of vapour. But it is a well known fact that cold is always generated during spontaneous evaporation; that is so say, that the water as it disappears carries off with it a quantity of caloric. It is well known, that when a wet body is exposed to the air, its temperature is lowered by the evaporation which takes place upon its surface. Hence, in warm countries water is cooled by putting it into porous vessels, and exposing it to the air. -The water penetrates through the vessels, evaporates from their surface, and carries off so much heat, as even in some cases to freeze the water in the vessel. Saussure observed, that the evaporation from the surface of melted snow caused it to freeze Saussure's Voyages dans les Alpes, iv. 263.

VOL. IV.