But, as we have said, a portion of carbonic acid gas is usually contained in air; this gas is exhaled from the earth, and collects in certain localities, rendering the air more impure than it otherwise would be; as a rule, however, the quantity of this gas in intermixture with the air we breathe—its lower strata—varies from three to eight parts out of a thousand in weight; in its pure state, it is immediately destructive to animal life; but thus diluted is in this respect innoxious, while to the vegetable kingdom it is necessary. With respect to water in a state of viewless vapour, the quantum in any given weight of air seldom exceeds 1½ per cent.
Now if, as most philosophers of the present day seem to consider, there is no chemical union of the gases constituting the atmosphere, it may be supposed that, according to their respective weights, a partial separation and subsidence of the heavier will take place, while the lighter will chiefly compose the upper portion—thus, for example, a stratum of nitrogen will surmount a stratum of mixed nitrogen and oxygen, and this a stratum of mixed nitrogen, oxygen, and carbonic acid. The following passage from the Penny Magazine will illustrate this theory: "A law is found to prevail in the mixture of gases and vapours, as universal as is that relative to their expansion arising from temperature, namely, that two gases in a state of mixture exercise no influence one upon the other, except communication of temperature, but that each is disposed in exactly the same way as it would be if the other were not present. Thus it is found, entirely contrary to all previous notions, that no pressure of dry air upon water exerts the least influence in preventing the formation of steam, which goes on exactly as if the space above were a vacuum, and continues until further evaporation is stopped by the pressure of the steam already created. It is found that no pressure of one gas can confine another in water; but that, supposing a bottle partly full of water, the gas confined in the water will escape to the surface, and distribute itself in precisely the same way as if the other gas were not present. By this it is not meant that the action, commonly called mechanical, cannot take place, or that a stream of hydrogen would not trouble the air; but only that the permanent settlement of one gas is not affected in any way by the presence of another, so long as no chemical action is excited.
"From this principle, Mr. Dalton,[3] taking into consideration the presumptions which exist against the chemical union of the ingredients of the atmosphere, infers, that the atmosphere does not altogether consist of the compound called air, but that the nitrogen atmosphere is higher than the oxygen atmosphere. In fact, if there be no chemical union, the above law of the mixture of gases requires us to allow that each is an atmosphere independent of the other, and that the two are most probably of unequal heights. From some considerations into which we cannot here enter, Mr. Dalton thinks that the actual pressures exerted by the oxygen and nitrogen are in the proportions of the volumes occupied by them, that is, as one to four; and concludes that the oxygen atmosphere extends to thirty-eight miles in height, that of nitrogen to fifty-four miles, that of carbonic acid to ten miles, and that of aqueous vapour to fifty miles. It must be observed, however, that the state of the carbonic acid of the atmosphere is very variable; that there is not the same quantity by night as by day, in moist weather as in dry; and that the higher strata of the atmosphere contain more of it than the lower, which may arise from a rapid absorption by the earth.
"Against the hypothesis just described, it might, perhaps, be asserted, that the air which Gay-Lussac brought down from a height of more than four miles was not found to differ from that of the earth's surface in the proportion of its oxygen to its nitrogen, which would be the case if the oxygen atmosphere diminished in density more than in proportion to the diminution of that of the nitrogen, or vice versâ."
[3] Phil. Trans., 1826.
Without attempting to settle the question as to whether the component parts of the air are in a state of chemical union, or only in a state of simple mixture, of this we are sure, that the air is essential to the existence of all organic beings which live on the surface of our planet, that is to animals and plants, though the relationship of each to the air is diverse. The following comparison between the two great groups of organic creation, which we copy from an admirable paper on the Progress of Organic Chemistry in the "Companion to the Almanack, 1849," shows the respective relationships of animals and plants to the elements around them, as well as their mutual balance and dependence:—
Is not the wisdom of God here manifest?—The vegetable kingdom is made auxiliary to the animal, and vice versâ.—Yet, how? by mysterious changes, appropriations, productions, and consumptions, upon which modern researches are from time to time throwing new light; or, in other words, unfolding to us new revelations of the power and wisdom of the Great Eternal, who has condescended to give us a spiritual directory, a guide to our feet, and a lamp to our path, by means of which we, who at best only see through a glass darkly, may be led into that glorious world where there are no clouds—where there is no night—where there is "no need of the sun, neither of the moon, to shine in it: for the glory of God" doth "lighten it, and the Lamb is the light thereof."