Let us examine each of those changes to which the ascending sap is subjected, in succession; and, first, with respect to the disengagement of superfluous water and carbonic acid, every one must have observed drops of water collected on the leaves of cabbages and other vegetables, when examined early in the morning. These are commonly supposed to be dew-drops, but are truly in great part the result of a kind of perspiration which is always taking place from the surface of plants. That this is the case, can be proved by covering a cabbage-plant with a bell-glass, and placing it in a room sufficiently heated to prevent the deposition of dew, when drops of water will be found equally to collect upon its leaves. These drops are not observed during the day, because the temperature is then commonly so high as to evaporate them as fast as they are transuded; but the fact is, that plants actually give off much more water during the day than night. The escape of carbonic acid is not so easily detected as that of water; it can, however, be proved, through the resources of chemistry. Unlike water, which is liberated both night and day, and indeed in greatest quantity during the latter period of time, carbonic acid is found to be disengaged during the night only. As long as plants are exposed to the light of the sun, their green parts liberate none of this gas. We have mentioned that when the ascending sap arrives into the leaves, it not only throws off superfluous water and carbonic acid, but likewise derives an additional quantity of nutriment from the atmosphere. The presence of light is necessary for this latter circumstance to take place. The nutriment which, under the influence of sunlight, it acquires from this source, is a substance named “carbon;” this substance is a constituent of carbonic acid, which is indeed composed of carbon and oxygen; carbonic acid is contained in the atmosphere in the proportion of one part in a thousand; the green parts of plants absorb it, and under the influence of light decompose it; the carbon is retained, but the oxygen is again liberated. We now may perceive the reason of the fact mentioned in the preceding paragraph: plants give out no carbonic acid during the day, because the superfluous carbonic acid of the ascending sap becomes decomposed under the influence of light, in the same way as that which has been absorbed from the atmosphere. A great many compound products are obtained from the vegetable kingdom. We need merely recall to the reader’s recollection starch, resin, camphor, bland and aromatic oils, bitter principles, colouring matters, the acids of the grape, the lemon, and the apple, &c. to assure him of this truth. All these different substances form themselves out of the sugar and gum of the ascending sap, together with the carbon absorbed under the influence of light.