"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:--