gen. Ammonia is absorbed by water, and it is therefore brought down to the earth by rain, where it forms a valuable manure for plants; its importance may be conceived when we state that the nutritious qualities of grain and other vegetable substances are mainly derived from the nitrogen contained in this aërial manure.
Watery vapour is constantly present in the atmosphere, though we can scarcely call it a constituent of air. Its presence can be easily demonstrated by putting some ice in a tumbler, for when the glass is sufficiently cool, the vapour will be condensed upon its outer surface in the form of dew.
We have resolved air into its component gases, and have thus exploded the old notion of air being an element.
Our investigations have brought to light certain bodies which may be justly considered elements, namely oxygen, hydrogen, nitrogen, and carbon. These substances have never yet been resolved into constituents, but we do not dogmatically assert that they are absolutely simple in their nature. We call them elements because we cannot prove them to be compounds, though it is not impossible that they may turn out to be such at some future time.
That a mixture of four dissimilar elements should produce the life-supporting atmosphere is a fact that may well excite our wonder. Who would suspect that the mild and genial air which envelopes
