There is hardly anything in vegetable physiognomy that makes so irregular and ineffaceable an impression upon a newly arrived person, as the sight of an arid plain thickly covered, like those near Cumana, New Barcelona and Cora, with columnar and candelabra-like divided Cactus stems."

We must all allow, that this fact of the growth of highly ligneous plants containing juices in arid plains, is not in favor of an hypothesis that considers vegetable mold as the true source of carbon for plants.

Besides the property of decomposing carbonic acid, vegetables have also the power of decomposing water; hence the source of hydrogen. At first sight we must imagine that there must be a marvellous energy in the chemical process of vegetation, when able to effect what the electricity of a powerful thunderstorm accomplishes only feebly and imperfectly: but when we reflect upon the various methods by which water can be decomposed, this feeling is somewhat limited. The metals - some at common temperature, others at a red heat, and the same, or more of them in contact with a strong acid; and, as it has been beautifully shown by Mr. Grove, by heat alone.

We know that this action must take place, from the fact that caoutchouc, wax, and oils, contain more hydrogen than oxygen; and we also know that water must be the only source of the hydrogen. The water is decomposed, the hydrogen is taken up into a plant with the green principle of the leaf, which diminishes in quantity when oxygen is absorbed. Plants containing water and carbonic acid, and evolving only a little oxygen, give an acid. - evolving more oxygen, they form a neutral substance. - evolving a large amount of oxygen they give us an oil.

Again, chemical analysis pointed out nitrogen as a constituent of plants, and for a long time it was a question how this nitrogen was obtained; later experiments, however, have shown that it has its origin in the ammonia which is always found in the atmosphere.

It was found that plants would grow in charcoal, or in calcined earth containing not a trace of carbon, if watered with rain water, and this because rain water contains more ammonia - hence its softness. So there are two forms in which this ammonia, so requisite for vegetation, may be found: as a gas existing in the atmosphere, (though this is seldom the case,) and held in solution by water which conveys it to the soil. Agriculturists find that the form in which it is given is of more importance than the actual quantity. Carbonate of ammonia is often found in large quantities; but it is a volatile salt, and for this reason a very considerable quantity of the ammonia it contains is volatilized and lost. The object of gypsum as a manure, is to produce that double decomposition by which is formed carbonate of lime and sulphate of Ammonia, a more stable compound.

The source of ammonia exhibits to us one of those beautiful and never ending cycles of mutual relationship upon which the mind of the real observer of nature always delights to dwell. Throughout the physical world, from its formation to the present time, there has never ceased to be a perpetual mutation of matter - a ceaseless, ever restless desire for change of form, and after some boundless wanderings, a turning back again, to undergo perchance the same work, though on a different subject - at a great distance from its former one - and after an inconceivably long interval had elapsed.

"Communion with nature awakens thoughts that had long lain dormant," enthusiastically exclaims the author of the "Cosmos." Surely this sentiment must find within a hearty echo, when for the first time we contemplate - actually by experimental demonstration - the imperishability and the indestructibility of matter: when, as in the case of the combustion of an organic substance with oxide of copper, the sugar, the volatile oil is de-destroyed, but its elements have assumed new forms, rendered cognizant to the senses by the balance.

Atmosphere, then passed into the composition of the flora of the gigantic vegetation of the coal-fields - the liberated oxygen in after ages uniting itself perhaps with a mineral, forming a sulphate - again to be reduced by organic matter to a sulphide - the carbonic acid freed again, passing off into oxygen by the vegetation of the oolite - taken up into the systems of the icthyosaurus - that this same oxygen, for what we know to the contrary, may even now be helping to carry on in us the vital process - is still at work to change again - to become as pure and free as it ever was, and not different from ourselves. - never to wear out or to decay, but while the world lasts to be pursuing a destiny predetermined before its existence by the Great Author of nature.

Ammonia is secreted from the body during life: it is a result of its putrefaction when dead. A thousand million of the human race, besides animals, annually die. How much nitrogen is thus given to plants, is a question that I think it is beyond the limit of science to answer. But it comes round to us again, though not exactly in the same form; for the plants supply the herbivora, who in their turn supply the carnivora. In some recent researches, I obtained from several coffins a nitrogenized compound called adipocere, rich in ammonia - in fact, an ammoniacal sap. In all cases, on opening a coffin, there was a powerful odor of ammonia; and as an illustration of the large amount of it in this adipocere, I may mention, that happening to have some in my pocket, on standing before a fife it began to melt; some ladies at the same time observing, "What a very strong odor of hartshorn there is!"

If all four can be represented by an oval, as some suppose - abovaomnia - somewhat on the same principle, the actions of nature might be represented by a circle. The excrement of plants afford another example of circular change. Besides those which are gaseous and solid, there are some liquid excrements which are excreted by the roots and absorbed by the soil: these fluids are very rich in carbon, so that the amount of that element which is furnished to the young plant by the humus is actually, by the secretions of the grown vegetable, returned again to it.