4. The smoke and other bodies which are continually carried into the air by evaporation, &c. are probably soon deposited again, and cannot therefore be considered with propriety as form. ing parts of the atmosphere. But there is another set of bodies which are occasionally combined with air, and which, on account of the powerful action which they produce on the human body, have attracted a great deal of attention. These are known by the names of matters of contagion.

That there is a difference between the atmosphere in different places, as far as respects its effects upon the human body, has been considered as an established point in all ages. Hence some places have been celebrated as healthy, and others avoided as pernicious to the human constitution. It is well known that in pits and mines the air is often in such a state as to suffocate almost instantaneously those who attempt to breathe it. Some places are haunted by peculiar diseases. It is known that those who frequent the apartments of persons ill of certain maladies, are extremely apt to catch the infection; and in prisons and other places, where crowds of people are confined together, when diseases once com. mence, they are wont to make dreadful havoc. In all these cases, it has been supposed that a certain noxious matter is dissolved by the air, and that it is the action of this matter which produces the mischief.

This noxious matter is in many cases readily distinguished by the peculiarly disagreeable smell which it communicates to the air. No doubt this matter differs according to the diseases which it communicates, and the substance from which it has originated. Morveau lately attempted to ascertain its nature; but he soon found the chemical tests hitherto discovered altogether insufficient for that purpose. He has put it beyond a doubt, however, that the noxious matter which rises from putrid bodies is of a com. pound nature; and that it is destroyed together by certain agents, particularly by those gaseous bodies which readily part with their oxygen. He exposed air infected by putrid bodies to the action of various substances; and he judged of the result by the effect which these bodies had in destroying the fetid smell of the air, The following is the result of his experiments.

1. Odorous bodies, such as benzoin, aromatic plants, &c. have no effect whatever. 2. Neither have the solutions of myrrh,.

benzoin, &c. in alcohol, though agitated in infected air. 3. Py. roligneous acid is equally inert. 4. Gunpowder, when fired in infected air, displaces a portion of it; but what remains still re. tains its fetid odour. 5. Sulphuric acid has no effect; sulphurious acid weakens the odour, but does not destroy it. 6. Vinegar diminishes the odour, but its action is slow and incomplete. 7. Acetic acid acts instantly, and destroys the fetid odour of infected air completely. 8. The fumes of nitric acid, first employed by Dr. Carmichael Smith, are equally efficacious. 9. Muriatic acid gas, first pointed out as a proper agent by Morveau himself, is equally effectual. 10. But the most powerful agent is oxymuriatic acid gas, first proposed by Mr. Cruikshanks, and now employed with the greatest success in the British Navy and Military Hospitals.

Thus there are four substances which have the property of de stroying contagious matter, and of purifying the air: but acetic acid cannot easily be obtained in sufficient quantity, and in a state of sufficient concentration, to be employed with advantage. Nitric acid may be attended with some inconvenience, because it is almost always contaminated with nitrous gas. Muriatic acid and oxymuriatic acid are not attended with these inconveniences; the last deserves the preference, because it acts with greater energy and rapidity. All that is necessary is to mix together two parts of common salt with one part of the black oxide of manganese, to place the mixture in an open vessel in the infected chamber, and to pour upon it two parts of sulphuric acid. The fumes of oxymu. riatic acid are immediately exhaled, fill the chamber, and destroy the contagion. Or the oxymuriate of lime, sold for the purposes of the bleacher, may be mixed with sulphuric acid, and placed in the infected apartment.

[Thomson's Chemistry.

SECTION. VI.

Variation of the Almosphere.

HAVING thus detailed the opinions of ancient and modern writers on the composition of the atmosphere, we shall now proceed to point out briefly the variation it exhibits at different elevations,

or under other circumstances, and which effects its weight, its pressure, or elasticity, and its temperature.

1. Weight and Pressure, or Elasticity of the Atmosphere; forming the Principle of Barometers, and the Ascent of Balloons.

ELASTIC fluids are distinguished from liquids by the absence of all cohesive force, or by their immediate tendency to expand when they are at liberty. Such are atmospheric air, steam, and gasses of various kinds; and the consideration of these fluids, in the state of rest, constitutes the doctrine of pneumatostatics, or of the equi. librium of elastic fluids.

That the air is a material substance, capable of resisting pres. sure, is easily shown, by inverting an empty jar in water; and by the operation of transferring airs and gasses from vessel to vessel, in the pneumatic apparatus used by chemists. The tendency of the air to expand is shown by the experiment in which a flaccid bladder becomes distended, and shrivelled fruit recovers its full size, as soon as the external pressure is removed from it, by the operation of the air pump: and the magnitude of this expansive force is more distinctly seen, when a portion of air is inclosed in a glass vessel, together with some mercury, in which the mouth of a tube is immersed, while the other end is open, and without the vessel; so that when the whole apparatus is inclosed in a very long jar, and the air of the jar is exhausted, the column of mercury becomes the measure of the expansive force of the air.

If the diameter of the tube, in an apparatus of this kind, were very small in comparison with the bulk of the air confined, the column of mercury would be raised, in the ordinary circumstances of the atmosphere, to the height of nearly 30 inches. But supposing the magnitude of the tube such, that the portion of air must expand to twice its natural bulk, before the mercury acquired a height sufficient to counterpoise it, this height would be 15 inches only. For it appears to be a general law of all elastic fluids, that their pressure on any given surface is diminished exactly in the' same proportion as their bulk is increased. If, therefore, the column of mercury in the vacuum of the air pump were 60 inches high, the air would be reduced to half its natural bulk; and for the same reason, the pressure of a column of 30 inches of mercury

in the open air will reduce any portion of air to half its bulk,since the natural pressure of the atmosphere, which is equal to that of about 30 inches of mercury, is doubled by the addition of an equal pressure. In the same manner the density of the air in a diving-bell is doubled at the depth of 34 feet below the surface of the water, and tripled at the depth of 68 feet. This law was dis. covered by Dr. Hooke; he found, however, that when a very great pressure had been applied, so that the density became many times greater than in the natural state, the elasticity appeared to be somewhat less increased than the density; but this exception to the general law has not been confirmed by later and more accurate experiments.

Not only the common air of the atmosphere, and other permanently elastic gases, but also steams and vapours of all kinds, appear to be equally subject to this universal law: they must, however, be examined at temperatures sufficient to preserve them in a state of elasticity; for example, if we wished to determine the force of steam twice as dense as that which is usually produced, we should be obliged to employ a heat 30 or 40 degrees above that of boiling water: we should then find that steam of such a density as to support, when confined in a dry vessel, the pressure of a column of 30 inches of mercury, would be reduced to half its bulk by the pressure of a column of 60 inches. But if we increased the pressure much beyond this, the steam would be converted into water, and the experiment would be at an end.

That the air which surrounds us is subjected to the power of gravitation, and possesses weight, may be shown by weighing a vessel which has been exhausted by means of the air-pump, and then allowing the air to enter, and weighing it a second time. In this manner we may ascertain the specific gravity of the air, even if the exhaustion is only partial, provided that we know the propor tion of the air left in the vessel to that which it originally contained. The pressure derived from the weight of the air is also the cause of the ascent of hydrogen gas, or of another portion of air which is carefied by heat, and carries with it the marks of a fire; and the effect is made more conspicuous, when either the hydrogen gas, or the heated air, is confined in a balloon. The diminution of the apparent weight of a body, by means of the pressure of the surrounding air, is also shown by the destruction of the equilibrium

between two bodies of different densities, upon their removal from the open air into the vacuum of an air-pump. For this purpose, a light hollow bulb of glass may be exactly counterpoised in the air by a much smaller weight of brass, with an index, which shows, on a graduated scale, the degree in which the large ball is made to preponderate in the receiver of the air pump, by the rarefaction of the air, lessening the buoyant power which helps to support its weight.

From this combination of weight and elasticity in the atmo sphere, it follows, that its upper parts must be much more rare than those which are nearer to the earth, since the density is every where proportional to the whole of the superincumbent weight. The weight of a column of air one foot in height is one twenty eight thousandth of the pressure; consequently that pressure is increased one twenty eight thousandth by the addition of the weight of one foot, and the next foot will be denser in the same proportion, since the density is always proportionate to the pressure; the pressure thus increased will therefore still be equal to twenty eight thousand times the weight of the next foot. The same reasoning may be continued without limit, and it may be shown, that while we suppose the height to vary by any uniform steps, as by distances of a foot or a mile, the pressures and densities will increase in continual proportion; thus, at the height of about 3000 fathoms, the density will be about half as great as at the earth's surface; at the height of 6000, one fourth; at 9000, one eighth as great. Hence it is inferred that the height in fathoms may be readily found from the logarithm of the number expressing the density of the air: for the logarithm of the number 2, multiplied by 10,000, is 3010, the lo garithms of numbers always increasing in continual proportion, when the numbers are taken larger and larger by equal steps. Hence we obtain an easy method of determining the heights of mountains with tolerable accuracy for if a bottle of air were closely stopped on the summit of a mountain, and, being brought in this state into the plain below, its mouth were inserted into a vessel of water or of mercury, a certain portion of the liquid would enter the bottle; this being weighed, if it were found to be onehalf of the quantity that the whole bottle would contain, it might be concluded that the air on the mountain possessed only half of the natural density, and that its height was 3000 fathoms. It ap