32 COMPOSITION OF THE ATMOSPHERE. 100 cubic inches of dry air weigh at 60° F. and 30 inches Bar., calculating from the experiments of Dumas and Boussingault of Prout of Regnault · 30'935 The second result is probably the most accurate, for it exactly corresponds with the density deduced from that of a mixture of oxygen and nitrogen in the proportions in which they occur in the atmosphere. A cubic foot of air, according to this result, weighs 536 96 grains at 60°. The weight of a given volume of air at 60° F., under a pressure of 30 inches Bar., is therefore only of that of an equal bulk of water at the same temperature. Owing to the greater solubility of oxygen than of nitrogen, rain water and melted snow always contain a larger proportion of oxygen than the air itself, amounting to about 34 per cent. of the air dissolved, or nearly one volume of oxygen to two volumes of nitrogen. This is a circumstance of great importance to aquatic animals, and one which could occur only in consequence of the air being a mechanical mixture and not a chemical compound of the two gases (94). In addition to oxygen and nitrogen the atmosphere contains a certain proportion of carbonic anhydride, a variable but minute trace of ammonia, traces of nitric acid, and of some compound of carbon and hydrogen, and frequently in towns a perceptible contained 20.88 per cent. of oxygen by volume, while air collected at the surface at the same time contained 2092. The air was collected in tubes of about 6 cubic inches in capacity, fitted with accurate stopcocks. They were exhausted previously to the ascent, and were filled with the air for examination by opening the stopcocks, which were again closed as soon as the charge had entered. In the extensive series of experiments of Regnault (Ann. de Chimie, III. xxxvi., 385), air was collected at different points of the earth's surface in glass tubes, drawn out to an open capillary extremity at either end, fig. 273. When a specimen of air was to be collected, one of these tubes was FIG. 273. attached by a flexible tube to a small pair of bellows, and by working the bellows a few times, the tube was filled with air of the locality. The capillary tubes were then drawn off and sealed, as at a and b, by momentary contact with the flame of a spirit-lamp, and the closed ends were protected from injury during the journey by small caps of glass tube fitted with corks. The analyses of the air thus obtained were executed by means of hydrogen, in a eudiometer of Regnault's contrivance. The same apparatus was used by myself in the analyses of the air collected by Mr. Welsh. Frankland found 20.96 of oxygen in air collected by himself from the summit of Mont Blanc. ESTIMATION OF AQUEOUS VAPOUR IN THE AIR. 33 amount either of sulphurous anhydride or of sulphuretted hydrogen. Aqueous vapour is of course also present at all times, although its amount is liable to extensive fluctuations. FIG. 274. (341) Estimation of Aqueous Vapour.-The amount of aqueous vapour at any spot may be ascertained by means of the hygrometer (194), or it may be determined by a direct experiment in the following manner. A bent tube, a, fig. 274, filled with pumicestone, moistened with sulphuric acid, is connected with a E vessel, E, of known capacity; suppose it be capable of containing 18 gallons of water. This vessel having been filled with water, is al lowed to empty itself slowly by opening a stopcock, f, which terminates in a tube bent upwards to prevent the entrance of air at the bottom; a known volume of air is thus drawn through the tube a, which retains all the moisture. If the weight of this tube be determined before commencing the experiment, and a second time after it is completed, the increase in weight will indicate the amount of moisture in the bulk of air operated upon. The temperature is ascertained by means of the thermometer, t, and the atmospheric pressure is obtained by an observation of the barometer at the time. The flow of water by the aspirator is rendered uniform during the whole course of the experiment by making the tube which conveys the air, sufficiently long to reach nearly to the bottom of the vessel, as shown by the dotted line which passes down from the central opening at the top. (342) Estimation of Carbonic Anhydride.-The quantity of carbonic anhydride in the air may be determined in the course of the same experiment. If the bulbs at 6 be filled with a strong solution of caustic potash (sp. gr. 125), and the tube c, with fragments of fused potash, the gain in weight experienced by the tubes b and c will indicate the quantity of carbonic anhydride which 34 ESTIMATION OF CARBONIC ANHYDRIDE IN AIR. FIG. 275. has been absorbed in the operation; the bent tube, d, is filled with pumice-stone moistened with sulphuric acid; it is not weighed, but is merely interposed as a measure of precaution between the aspirator E, and the tube c, to prevent any accidental trace of moisture from passing backwards into c. The bulbs seen at b are to be filled with solution of hydrate of potash to the extent shown in the enlarged drawing, fig. 275. This form of apparatus was contrived by Liebig. It is in continual requisition in the laboratory, for the purpose of absorbing gases which are transmitted through it; by placing it a little on one side, the gas is made to bubble up successively through each of the three lower bulbs, besides being brought thoroughly into contact with the liquid in the narrow portions of tubing which connect the different bulbs together. This simple contrivance has added greatly to precision in experiments of this kind.* The proportion of carbonic anhydride in the atmosphere varies from 3 to 6 parts in 10,000 of air. De Saussure found that within these limits its amount is lessened after rain, owing to the solvent action of the descending shower, which carries a portion of the gas with it to the earth. It increases during a frost, and diminishes when a thaw sets in. During the night it increases, and diminishes again after sunrise. It is less in amount over large bodies of water than over large tracts of land. portion of carbonic anhydride is less liable to vary on elevated mountains, where it is generally more abundant than in the plains. It is also more abundant in densely populated districts than in the open country. In inhabited dwellings, and in rooms for public assemblies, the proportion of carbonic anhydride may, however, greatly exceed the normal amount. The pro The quantity of ammonia and nitric acid in the atmosphere is materially diminished after long-continued and heavy rains. Occa *Pettenkofer estimates the quantity of carbonic anhydride in air by agitating a given volume of the air for trial with a measured amount of limewater of known strength,. The lime-water used for this purpose is graduated by the alkalimetric method, by means of a standard solution of oxalic acid. The carbonic anhydride neutralizes and precipitates a certain quantity of lime in the form of chalk, and the quantity of lime which remains in the solution after the experiment is again determined by the solution of oxalic acid. The difference in the quantity of lime before and after its action upon the air enables the operator to calculate the proportion of carbonic anhydride with great accuracy. sionally, from local and accidental circumstances, other gases and vapours are also met with. The air of towns contains in addition certain organic impurities in suspension. Dr. Angus Smith has attempted to estimate their amount by measuring the quantity of a very dilute solution of permanganate of potassium of known strength which a given bulk of air will deprive of colour.—(Q. J. Chem. Soc. xi. 217.)* The average composition of the atmosphere in the climate of England may be approximatively stated as follows, in 100 parts by volume :— Average Composition of the Atmosphere. § I. WATER. H2O=18; Atomic and Mol. Vol. of Vapour or HO 9: Sp. Gr. as Vapour 0622, as Liquid 1'000, as Ice o'918. (343) ON the uses of water it is almost needless to enlarge, for they are universally felt and appreciated. In each of its three physical conditions, the blessings which it confers upon man are inestimable. As ice, it furnishes in northern lands for months together, a solid bridge of communication between distant places: in the liquid condition, it is absolutely necessary to the existence of vegetable and animal life; in this shape, too, it furnishes to man a continual source of power in the flow of streams and rivers; it supplies one of the most convenient channels of communication between places widely separated; and further, it is the storehouse of countless myriads of creatures fitted for use as food in the *If air which has been scrupulously freed from carbonic anhydride be passed over a column of pure ignited oxide of copper, traces of carbonic anhydride are always obtained, owing to the oxidation of some combustible compound of carbon. In the junctions of the apparatus employed for this experiment the use of cork and caoutchouc must be avoided (Karsten), or otherwise the carbonic anhydride might be derived from them. 36 GENERAL PROPERTIES OF WATER. state of vapour, as applied in the steam-engine, it has furnished a power which has in later years done more than any other physical agent to advance civilization, to economize time, and to ameliorate the social condition of man. In each and all of these points, if rightly considered, we must perceive the entire adaptation of this wonderful compound to the ends which it was designed by the Creator to fulfil. Glancing at the physical condition of our planet, we cannot fail to be impressed with the important effects produced by the movements of water at periods anterior to the existence of man, as well as in more recent times. To such causes must we refer the formation of sedimentary rocks and their arrangement in successive strata upon the surface of the earth: even now, observation shows that denudation is proceeding at some points, elevation and filling-up of hollows at others; whilst the accumulation of drift and a variety of other extensive geological changes must be traced to the same ever-acting and widely operating agency. It may further be observed that there is no form of matter which contributes so largely as water to the beauty and variety of the globe which we inhabit. In its solid state we are familiar with it in the form of blocks of ice, of sleet and hail, of hoar-frost fringing every shrub and blade of grass, or of snow protecting the tender plant, as with a fleecy mantle, from the piercing frosts of winter. The rare but splendid spectacles of mock suns, or parhelia, are due to the refractive power of floating spicule of ice upon the sun's rays. In its liquid condition, as rain or dew, it bathes the soil; and the personal experience of all will testify to the charm which the waterfall, the rivulet, the stream, or the lake, adds to the beauty of the landscape; whilst few can behold unmoved the unbounded expanse of ocean, which, whether motionless, or heaving with the gently undulating tide, or when lashed into foam by the storm that sweeps over its surface, seems to remind man of his own insignificance, and of the power of Him who alone can lift up or quell its roaring waves. In vapour how much variety is added to the view by the mist or the cloud, which by their everchanging shadows diversify, at every movement, the landscape over which they are flitting; whilst the gorgeous hues of the clouds around the setting sun, and the glowing tints of the rainbow, are due to the refractive action of water and watery vapour upon the solar rays. Properties.-At the ordinary temperature of the air, water, when free from admixture, is a clear, colourless, transparent liquid, destitute of taste or smell. At temperatures below 32° it freezes |