Irritability, denotes that essential property of animals, in consequence of which their fibres, on being touched, contract or become short, in a greater or less degree.

Chemists have lately endeavoured to shew, that all animal and vegetable irritability originates from the oxygen, which is inhaled by the lungs, or respired by the leaves, or absorbed by the roots. And as respiration is every minute requisite to the support of animal life, Dr. Darwin conjectures that something immediately necessary to their existence, is acquired by the lungs of animals from the atmosphere, rather than from the food they digest ; and that this nameless something, perhaps oxygen, is mixed with the blood, and again separated from it by the spinal marrow, after having undergone certain changes in the course of its circulation, or secretion. In a similar manner, he considers it as probable that the spirit of vegetation may be derived from that source, namely, the un-combined oxygen of the air, which is respired by the upper surfaces of the leaves, and not from that which is absorbed by roots, in a more combined state: farther, that this oxygen is again separated from the juices by the sensorium, or brain, of each individual bud, after having undergone some change in its passage through the secretory vessels.

A long-continued, unusual, or unnatural stimulation of vegetable fibres, by an increase of heat, ex hausts the spirit of vegetation : hence a slighter degree of cold will destroy such fibres; because, after having been excited for a considerable time by a more powerful stimulus, they will cease to act on the application of one that is weaker, so that in consequence of hot days, tender plants are more liable to perish from the coldness of the night. For this reason, gardeners 0inthe more northern climates, shelter both the flowers of apricots and the tender vegetables, during the spring-frosts, from the meridian sun, as well as from the destructive cold of the night, which is generally the most intense about one hour before sun-rise.

In the hot days of the month of June, 1798, Dr. Darwin twice observed several rows of garden-beans to become quite sickly; and some of them even died, in consequence of being irrigated for one or two hours with water from an adjoining canal. This circumstance he attributes to the sudden application of cold, after the plants had been greatly enfeebled, and deprived of their irritability by the excessive heat of the season, rather than to the too copious watering of the dry soil. Some vegetables, however, such as strawberries, onions, etc. which were equally exhausted, are less liable to be thus injured, and even receive benefit from occasional irrigation in dry and sultry weather.

On the contrary, the spirit of vegetation acquires additional vigour, if plants have been exposed for a considerable time to a less than usual degree of heat; but, if they be suddenly removed from a cold to a warm place, they will experience a similar fate with those hapless persons, to whose frozen limbs sudden heat was imprudent-ly applied : thus, too great increase of action occasions inflammation, which is generally succeeded by mortification and death. This fact has been ascertained and confirmed by the experiments of M. Van Uslar, who kept two species of the spurge or wartwort, namely, the Euphorbia Peplus, and Esula, L. secluded from the light and heat, with a view to render them more irritable : on exposing these plants afterwards to a meridian sun, they became grangren ous, and in a short time decayed.

The increased, or diminished, degree of irritability in plants, is by Dr. Darwin attributed to their previous habits, with respect to-the stimulus of greater or less heat. Thus, the periods at which vegetables thrive in the spring, appear to be greatly influenced by their acquired habits. as well as by their present sensibility to heat: hence potatoes will germinate in a much cooler temperature during the spring than in autumn ; hence also, the vernal months are the most fa-voilrable to the process of making good malt; because the barley will then sprout with a less degree of heat than at any other season.

The irritability of plants has been discovered to be greater in the morning than at noon, and less in the evening than at mid-day ; though it is considerably augmented during cool and rainy -weather. In a similar manner, the limbs of animals acquire a greater sensibility of heat, after having been exposed to the cold : thus, the hands, after immersing them for a short time in snow, glow with warmth on entering the house;—and the late celebrated Spallanzani observed several animals and insects, that conceal themselves under ground in a torpid state during the winter, and enjoy the genial warmth of the spring, again to disappear at a season when the heat of the atmosphere was much higher than on their first emerging from their subterraneous abode.

From these considerations, Dr. Darwin infers, that such plants as are sheltered in a warm room, during the winter, ought to be occasionally exposed to a cooler atmosphere, in order to increase their irritability; otherwise their growth in the succeeding spring will be much retarded. For the same reason, the continual vicissitudes of the air and weather are essentially necessary to mankind ; as the frequent changes of heat and cold have an obvious tendency to preserve or restore their irritability, and consequently the activity of the system. Hence not only the health and energy of men are more conspicuous in our variable climate, but their longevity is comparatively greater than on those tropical continents, which experience both a more considerable degree of heat, and a more steady atmosphere.