sun, it gives off oxygen, by decomposing the carbonic acid; whereupon the carbon remains behind in the interior of the leaf in a solid state. Although the nature of the air thus extricated can only be determined by a chemist, yet the extrication itself can be easily seen by any one who will plunge a leaf in water and expose it to the sun; for bubbles of oxygen will be seen to form themselves upon the surface of the leaf. But, if the same leaf be observed in the total absence of solar light, there will be little or no extrication of air, and what little is given off will be found to be carbonic acid, which plants exhale at all times in small quantities; oxygen, however, which was before expelled, is inhaled. Hence plants decompose carbonic acid during the day, and form it again during the night, the oxygen they inhale at that time entering again into combination with their carbon; and, during the healthy state of a plant, the decomposition by day, and recomposition by night, of this gaseous matter, is perpetually going on. The quantity of carbonic acid decomposed is in proportion to the intensity of the light which strikes a leaf, the smallest amount being in shady places; and the healthiness of a plant is, cæteris paribus, in proportion to the quantity of carbonic acid decomposed; therefore, the healthiness of a plant should be in proportion to the quantity of light it receives by day.

68. But, while this is true as a general axiom, it is necessary to observe that some plants are naturally inhabitants of shady situations, and are so organised as to be fit for such places and for no others plants of this description will not endure full exposure to the sun; not because an abundant decomposition of carbonic acid is otherwise than favourable to them, but because their epidermis allows the escape of water too freely by insensible perspiration, under the solar stimulus.

69. The mere fact of plants absorbing fluids from the earth, would render it probable that they have some means of parting with a portion of it by their surface; but that they do perspire is susceptible of direct proof, and is by no means a mere matter of inference.

70. We do not indeed see vapour flying off from the surface of plants; neither do we from that of animals, except when the air is so cold as to condense the vapour; yet we know that in both cases perspiration is perpetually going on, and it would appear that in plants it takes place more abundantly than in animals. If a plant covered with leaves is placed under a glass vessel, and exposed to the sun, the sides of the vessel are speedily covered with dew, produced by the condensation of the insensible perspiration of the plant. If the branch of a plant is placed in a bottle of water, and the neck of the bottle is luted to the branch, so that

no evaporation can take place, nevertheless the water will disappear; and this can only happen from its having been abstracted by the branch which lost it again by insensible perspiration. Hales, an excellent observer, devised many experiments connected with this subject *; among others the following, which he relates thus: — August 13. In the very dry year 1723, I dug down 2 feet deep to the root of a thriving baking pear tree, and laying bare a root half an inch in diameter (fig. 7.). I cut off the end of the root at i, and put the remaining stump (in) into the glass tube dr, which was an inch in diameter, and eight inches long, cementing it fast at r; the lower part of the tube d z was eighteen inches long and a quarter of an inch diameter in bore. . . . Then I turned the lower end of the tube (z) uppermost, and filled it full of water, and then immediately immersed the small end z into the cistern of mercury at the bottom, taking away my finger which stopped up the end of the tube z. . . . . The root imbibed the water with so much vigour, that in six minutes' time the mercury was raised up the tube d z as high as z, namely, eight inches. The next morning at eight o'clock the mercury was fallen to two inches in height, and two inches of the end of the root i were yet immersed in water. As the root imbibed the water, innu

• See Vegetable Statistics, London, 1727.

merable air bubbles issued out at i, which occupied the upper part of the tube at r as the water left

it." On another occasion he planted a sunflower 3 feet high in a garden pot, which he covered with thin milled lead, cementing all the joints so that no vapour could escape except through the

sides of the pot and through the plant itself; but providing an aperture, capable of being stopped, through which the earth in the pot could be watered. After fifteen days, viz., from July 3. to August 8., he found, upon making all necessary allowances for waste, that this sunflower plant 3 feet high, with a surface of 5616 square inches above the ground, had perspired as follows:

Ounces Avoirdupois.

In twelve hours of a very dry warm day 30,

and that when the dew was copious, or there was rain during the night, the plant and pot were increased in weight two or three ounces. Other persons have instituted other experiments of a similar nature, the result of all which is, that the insensible perspiration of plants is very considerable.* Hales says his sunflower perspired

* The amount of this force is strikingly illustrated by the following circumstance recorded by the late Mr. Braddick. "One experiment I will mention, as it may serve to show the great power of the rising sap in the vine, while its buds are breaking. On the 20th of March, in the middle of a warm day, I selected a strong seedling vine five years old, which grew in a well prepared soil, against a south-west wall; I took off its head horizontally with a clean cut, and immediately observed the sap rising rapidly through all the pores of the wood, from the centre to the bark. I wiped away the exuded moisture, and covered