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."

2. Its Electrical Condition.—With the electrical changes in the state of the atmosphere are connected various transitions from heat to cold, dew, rain, hail, snow, clouds, winds, thunder-storms, auroræ boreales, haloes, parhelia, etc. In this point of consideration the atmosphere bears upon organic life. The solid globe itself may be regarded as a vast electrical or galvanic apparatus, and all the vital functions of animal bodies involve electric or galvanic phenomena. Magnetism appears to be essentially identical with electricity; both are most intimately connected with light and heat; and not only electric sparks, but all the phenomena of electricity, can be obtained from a common magnet. It is not here our purpose to enter into a philosophical treatise on electricity and galvanism; a little consideration, however, will lead us to infer that the general diffusion of this subtle electric fluid, if the term fluid be at all applicable, is essential to the maintenance of organic life, and that electrical changes are perpetually taking place in organic bodies. This fluid is derived from the earth, or its circumambient atmosphere, and its currents obey definite laws, which have been elucidated by the experiments of some of the most profound philosophers of modern times.

After what we have briefly said respecting the electric condition of our globe, and the electrical phenomena which are involved in the processes of the vital functions, to say nothing of electro-magnetism, it must surely be apparent, that even as the air is in itself necessary to our existence, so the