III.

ON AQUEOUS METEORS.

DAILY experience proves to us that the hygrometric state of the atmosphere is continually varying. During storms, water is precipitated from the clouds in torrents, or else it is slowly deposited in the state of dew; sometimes the air is so dry that wood is warped, and then water evaporates with great rapidity in open vessels. We designate all these phenomena by the collective name of hydrometeors, a word derived from the Greek dwg, water, and which signifies aqueous meteors.

GENERAL REMARKS ON GASES AND VAPOURS.

-Water, in passing into the state of an aëriform body, occupies much greater space than that which it filled while it remained in a liquid state. So that water can exist under two different forms, having nothing in common except the extreme mobility of the component molecules, which are separated and move easily one over the other. In solid bodies, on the contrary, a greater or a less effort is necessary to separate the molecules of which they are composed. The particles of water have, however, a tendency to approach and form little spherical masses. In gases, properly so called, this tendency does not exist, and all the molecules are mutually repulsive; hence results an expansive force for the entire gas. It is also almost an impossibility to approximate or to separate the molecules of water while in the liquid state; whilst a volume of gas, however small it may be, will always entirely fill a vessel of any capacity. Place a bellglass on the plate of an air-pump, and make as perfect a vacuum as possible; there will, nevertheless, be air in every part of the glass. The following experiment proves that the air is actually dilated; to wit, if we place a tied and

flattened bladder under the receiver, it will swell out in proportion as the vacuum is made, but will return to its former condition, as soon as the air is allowed to re-enter the glass. There is, indeed, an equilibrium established between the repulsive forces of the molecules of air contained in the bladder, and those which fill the receiver.

We may convince ourselves of the truth, without the use of philosophical apparatus. Take a small quantity of sulphuret of potassium and pour on it sulphuric acid, a gas will be liberated that smells powerfully of rotten eggs; however small may be the volume of the gas, it will not fail to fill a large room. It is the same with a drop of ether, when evaporated; and philosophy shews that this law is applicable to all gases without exception.

If the attraction of the earth did not neutralise this expansive force of the air, it would escape into space, and our globe would have no atmosphere. Attraction here plays the same part, as do the sides of the vessel in the experiment that we have related. Air, when subjected to this force of attraction, becomes a heavy body, like all others on the surface of the globe. So also a balloon, when void of air, is lighter than the same balloon when full.

Although the particles of gases incessantly tend to separate from each other, we may nevertheless easily diminish the volume of a certain mass of air. A bladder becomes smaller when compressed. A piston may be driven into a hollow cylinder of the same diameter, although it is hermetically sealed; but, as soon as we cease pressure on the piston, the air expands and drives it back. Provided the aëriform bodies do not pass into the liquid state, the spaces occupied are inversely proportional to the compressing forces; that is to say, under a double, triple, quadruple, &c. pressure, they occupy one-half, one-third, or one-fourth of the primitive space.

PHYSICAL COMPOSITION OF THE ATMOSPHERE. -Each of the molecules of which it is composed, by virtue of its gravity, exercises a pressure on the molecules situated beneath it; this pressure is added to their proper gravity, and contributes, in combination with the action of the terrestrial globe, to retain them around it. In a vertical column of the air, strata of greater density are found near the ground; this density diminishes in proportion as we ascend, because the portion of the atmosphere, placed beneath the observer, does not exercise any pressure on those portions which are placed at a level with him. The barometer, by which this pressure is measured, is lower at the summit than

at the foot of a mountain; and so intimate a relation exists between the pressure and this height, that the difference of level of those spots may be deduced from the difference in the length of the barometric columns observed simultaneously at these two stations.

The more the pressure diminishes, the more does the air tend to dilate; so that, at first sight, it would seem that the atmosphere must extend to a very great distance. We might imagine that it is not till the distance of several myriametres that the density could be sufficiently reduced to be entirely neglected. Experience has not taught us what becomes of those particles of air, whose density would be infinitely more feeble than it is at the surface of the earth. If their expansion were indefinite, they would be diffused into celestial space, and each of the bodies moving there would form an atmosphere by attracting them to itself. Astronomical observations do not favour this hypothesis, and it is probable that the atmosphere of the earth is limited. The distance of the limit is not yet well known; we merely know that, at the height of about seven myriametres, the rarity of the air is such that we may consider this as the limit of the atmosphere.*

*M. BIOT has lately published some learned researches on the physical constitution of the atmosphere; they have led him to a condition, which assigns a higher limit to the terrestrial atmosphere. He has borrowed the elements of his calculations from three series of barometric, thermometric, and hygrometric observations, made at successive stations by MM. GAYLUSSAC, HUMBOLDT, and BOUSSINGAULT.

M. GAY-LUSSAC ascended in a balloon, in Oct. 1803, to a height of 6977 metres above the observatory at Paris. The number of intermediate observations is twenty-one.

In the month of June, 1802, M. de HUMBOLDT made observations at five successive stations, as he ascended from the plains at the foot of Chimboraço to the top of the mountain. The first station was 2418, the last 5879 metres above the level of the sea.

Finally, in 1827, M. BOUSSINGAULT made three series of meteorological observations in his ascents up Chimboraço and Antisana, to the heights of 5900 and 5400 metres above the level of the Pacific Ocean. The Chimboraço series comprehends eight elevated stations, commencing at the height of 2700 metres. Each Antisana series comprehends nine, commencing at 2500 metres.

In order to deduce the height of the atmosphere from these observations, M. BIOT first reduces the barometric columns of the different stations to zero; then he reduces them all to the lowest weight, by calculating the correction which each requires from the relative elevation of the station. Dividing all these columns thus reduced by the lower column, he obtains the successive pressures, in fractions of the lower pressure taken as unity.

M. BIOT then deduces the densities corresponding to these pressures, from the concomitant temperatures of the air, admitting with M. GAYLUSSAC, that the air diminishes th of its volume for every centigrade degree of cold. He takes account at the same time of the tension of the aqueous vapour. The densities thus obtained are compared with the lower density of their proper unity, in the same way as was done for the pressures.

DIFFERENCES BETWEEN GASES AND VAPOURS. -Aëriform bodies are naturally divided into two classes; some always remain in the gaseous or elastic state, and are called gases or aëriform bodies,3 others, under the influence of various circumstances, pass into the liquid state, and are termed vapours. Among the agents which determine this change, temperature and pressure must occupy the first rank. Bend a common barometric tube, ABC (pl. 11. fig. 3), so that the branch BC is parallel to the branch AD, and close it at C; adapt a scale to the branch BC, which shall indicate the number of cubic millimetres contained in CE, or any portion of the tube CB; adapt in like manner a scale divided into millimetres to the branch AD, dry the interior of the tube by placing it in connexion with a vessel containing anhydrous sulphuric acid, then pour mercury into the longer branch so that it shall be in equilibrium at D and E. The quantity of air contained in CE is no longer in communication with the atmosphere, but is subject to a pressure that is indicated by the height of the barometer. If we pour mercury into the long branch until the column is at F, it will only ascend as far as G in the short trough. Draw through the point G the horizontal line GH, and the measure of the pressure will be obtained by adding the length GH to that of the barometric column observed during the time of the experiment. If FH is equal to the length of the barometric

We thus obtain the coexistent values of these two elements, for all the points of the aerial column, where the stations have been established. Taking, then, the pressures as abscissæ, and the densities as ordinates, M. BIOT finds that the curve, which passes through all the stations, is sensibly a straight line. He hence concludes that the decrease of temperature goes on incessantly accelerating to the highest stations to which we have been able to attain. Thus, according to M. BIOT, we must not conclude that further on, and in the inaccessible regions of the atmosphere, this decrease begins to be reduced; and, among the hypotheses that may be adopted, the most favourable to a very elevated atmosphere, will then be that of a constant decrease beyond the height of 6977 metres, the upper limit of the aerostatic stations of M. GAY-LUSSAC. At present, beyond that elevation, M. BIOT substitutes for the real atmosphere a fictitious atmosphere, having at this height the same density, the same degree of pressure, the same heat, and the same local decrease of temperature as the true atmosphere; but subject further to the arbitrary condition that the decrease remains constant, and such as M. GAY-LUSSAC has observed it. Such a condition, joined to the laws of equilibrium, completely defines it; and from the physical elements of the stratum, where it commences, its total height joined to that of this stratum, is 47346 metres. Now, in the real atmosphere, the decrease of temperature being further accelerated beyond 6977 metres, he finds that its limit is lower than that of the fictitious atmosphere, or at 47000 metres. The equatorial series of MM. de HUMBOLDT and BOUSSINGAULT, give even 43000 metres for this upper limit. (Vide Comptes rendus de l'Académie des Sciences, t. viii. p. 91;. and t. ix. p. 174 [1839].-Additions à la Connaissance des Temps de 1841.Memoires de l'Académie des Sciences, t. xvii.—Astronomie Physique, t. i. p. 165.)-M.

3 Vide Note c, Appendix II.

column, the air contained in GC will be subject to the pres sure of two atmospheres; then CG will be equal to the half of CE, and the air will occupy a space one-half less than that which it occupied under the pressure of one atmosphere. By increasing the pressure, we shall succeed in establishing Mariotte's law, already announced, p. 59, that the spaces occupied by gases are inversely proportional to the pressures.

Dry air obeys this law under every pressure hitherto tried. If the air is moist, it will follow the law under feeble pressures; but, under high pressures, the spaces will become less than they would have been had the air been perfectly dry; for, under such circumstances, a portion of the vapour of water condenses and passes into the liquid state, and drops of water are even observed within the tube CE.

The difference between gases and vapours may be demonstrated by another experiment. Take three barometers which have been well boiled, and which correspond well. Designate the three instruments by the letters A, B, and C. Divide the barometric chambers of B and Cinto parts of equal capacity. Send up a bubble of dry air into the vacuum of B. The dilatation of this air will lower the column of B, which will remain lower than that of A. The difference will give the measure of the elasticity of the gas at this temperature. Send up a drop of liquid into the barometric chamber of C, it will be converted into vapour, which will depress the mercury; and the quantity of this depression, compared with A, will give the tension of the vapour of water at that temperature. Plunge the two tubes B and C vertically into a mercury cup, their mercurial columns will always be shorter than that of A; but the difference between A and B will continue increasing in proportion as the air is more compressed-a proof that its elasticity increases, while the difference between A and C remains invariable. The vapour, therefore, of water has always the same elasticity in a filled space, whether this space be great or small; for, as soon as this space is contracted, a part of the vapour of water passes into the liquid state. It is only while the space is not saturated that the vapour acts as a gas, until the space is sufficiently contracted to be saturated.

Temperature produces the same effects as pressure. Suppose the three barometers to be placed in a situation in which the thermometer stands at 20°. Suppose further, that the mercurial column A is 758mm long; those of B and C, 740mm; the elasticity both of the air and the vapour will be equal to 18mm. Let the instruments be carried to a