not a compound, that has hitherto been an enigma in Science, ever since the fact was discovered.

The more so that, neglecting to take into account that "time is the creation of Man," it was judged from a few analyses extending over a small number of years, that the proportion of oxygen and nitrogen in the air is invariable, and never has varied!

If only a mixture, and not a definite compound, how is it, people ask, that its composition remains fixed?— how is it that it shows the same quantities of oxygen and nitrogen in whatever part of the Earth it is analyzed, and at whatever height above the sea the sample of air is taken?

All this is explained, of course, by allusion to the law of diffusion of gases. But it must not be forgotten that changes in nature are very gradual, and very slow in comparison with the short, rapid span of Man's existence.

Several eminent men have already supposed, as we have seen, that the composition of the atmosphere is changeable. A slight practical glimpse of such a fact occurred in the analyses made by Levy, a Danish chemist, of air lying over the waters of the ocean. between Havre and Copenhagen; and I myself made similar experiments with air taken at the surface of the sea many miles from the coast of Flanders. Such air contains somewhat less oxygen than that which is collected on the land. This was explained by the fact that oxygen gas is slightly more soluble in water than is nitrogen; and free oxygen is required for the respiration of aquatic animals.

The same slight deficiency of oxygen should be found in air lying over vast inland lakes; for, the air extracted from fresh water, by boiling, contains 33 per cent. of oxygen, instead of 21 (the proportion in atmospheric air). But the constant motion of the air keeps its composition very similar in all its parts, like that of a solution of sugar constantly stirred by a glass rod. Then, the nature of Nitrogen formed a subject of uncertainty. Here is a substance which, combined with oxygen, forms one of the strongest acids, and, combined with hydrogen, one of the most powerful alkalies.

Now, other substances combine both with oxygen and hydrogen, but, as Berzélius remarks, always to form acids (Traité de Chimie, vol. i. p. 146, Belgian edition). It is true that phosphorus and arsenic imitate nitrogen in this respect, but they form nothing comparable for energy with nitric acid and ammonia.

Berzélius did not perceive that nitrogen is essentially neutral, or inert; that is why it must have formed the original atmosphere of the Earth. Hence, in nitric acid all the electro-negative properties of oxygen are apparent, and in ammonia all the electro-positive properties of hydrogen. In neither case does the neutral nitrogen interfere. It is the only element of this absolutely neutral nature. Hence, also, all the compounds of nitrogen have to be formed indirectly, by roundabout processes.

We have no facts in the whole range of Chemistry more striking than that the two substances ammonia

and nitric acid, so essentially opposite in character, are readily convertible one into the other. I have found, for instance, that ammonia oxidized in the cold by a solution of permanganate of potash, forms nitrite and nitrate of potash; and it has long been known that when zinc, or tin, is dissolved in nitric acid ammonia is found in the solution.

For many years it has been supposed that the nitrogen of the atmosphere plays an important part in the process of "nitrification," and many theories have been put forth to explain it-porous bodies, catalysis, electricity, bacteria, etc.-but we may safely assert that it is still unexplained.

In several experiments I made, years ago, with the view of thus forming saltpetre artificially, I never obtained any nitrates unless some ammonia-yielding substance was present, and I look upon the phenomenon of nitrification as due to the slow oxidation of ammonia in nature. The process is universal; it occurs constantly, everywhere; but it is only in those parts of the globe where rain is scarce that the resultant nitrates are easily discovered, or where they effloresce from the soil. In all other places they are washed away by the rain as fast as they are produced, and find their way into the rivers.

Now, ammonia is not only a volcanic product (like carbonic acid), but an organic residue-a secretion that has found its way into the superficial strata of the Earth ever since life appeared upon the globe. Ammonia is a poison to plants, but nitrates are absorbed by all

vegetables, and I have convinced myself that ammonia is converted into nitric acid before its nitrogen can enter the plant, and that ammonia will kill the plant unless this conversion can take place.

In the earlier ages of the globe, there could have been no nitric acid, nor even ammonia; but when the Earth had cooled sufficiently-long before life appeared— ammonia could exist in the volcanic products as it does at the present day, but no nitric acid.1 Later still, nitric acid formed from ammonia (“nitrification") was produced, and plant-life became possible.

Ammonia, like carbonic acid, must therefore be looked upon as a volcanic product; and, when organized beings decay, their nitrogen and carbon return to nature as ammonia and carbonic acid.

Hence it is evident that atmospheric nitrogen takes no part in the important process of nitrification, unless it be that which is directly converted into nitric acid during combustion of various substances in the air, or by the lightning flash, and which may, after all, be due to the ammonia that is always present in the air.

1 Ammonia has recently been found in the mineral Apophyllite, to the extent of 0.03 to 0.5 per cent.

CHAPTER VII.

Definition of the Atmosphere of the Present Day-Transparency -Spectral Lines of Oxygen not given by the Sun's Atmosphere-Observations and Experiments of Janssen, PiazziSmyth, Langley, Dewar, and Faraday.

A POPULAR writer on Science has said: "The Earth we inhabit is surrounded by an atmosphere of air, the height of which is known to be at least forty-five miles. It presses upon the Earth with a weight equal, at the level of the sea, to about 15 lbs. on every square inch of surface. As we ascend high mountains, this weight becomes less; as we go down into deep mines, it becomes sensibly greater. We breathe this atmospheric air, and without it we could not live many moments. It floats around the Earth, being in perpetual motion; and according to the swiftness with which it moves, it produces gentle breezes, high winds, or terrible tornadoes."

It is hardly possible to give, in fewer words, a general definition of the Earth's atmosphere.

The transparency of the air, cæteris paribus, must increase with the height above the Sea; but this increased transparency shows itself in a very remark