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PROP. I.

If the Supply of the IRRITABLE PRINCIPLE to the moving fibre, be equal to the expenditure by the action of ftimuli, the fibre is then faid to be in a state of TONE.

PROP. II.

If the expenditure exceed the supply of THIS PRINCIPLE to the fibre, it is faid to be in a state of EXHAUSTION *.

PROP. III.

But if the fupply given to the moving fibre exceed the expenditure, the fibre is then faid to be in a state of ACCUMULATION.

Thele propofitions have been proved in the foregoing part of the work?

SECT.

SECT. XXXVIII.

OF COLD.

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DURING the winter, by the abfence of the ftimulus of heat, and in part of light, plants and many animals become torpid, the organs of circulation, and of nutrition, perform their functions but languidly, and life itself appears suspended. In confequence of the diminish ed action of thefe fiimuli, THE IRRITABILITY accumulates, and manifefts itself at the return of Spring. A flight degree of heat then produces powerful effects upon the fibres thus delicately irritable. Animals, which had concealed themselves under ground, even when the cold is greater than in autumn, venture forth from their fubterraneous retreat, trees and plants put forth their leaves and bloffoms, and birds, and animals, and man himself, is fenfible of the stimulus of heat from the return of Spring, his fibres being rendered more irritable by the winter's cold.

Dr. HALE, in his Vegetable Statics, relates that he cut down a vine, and cemented to its mutilated stump glass tubes, each 7 feet long, and one fourth of an inch diameter,

7

diameter, with brafs caps, by which they were screwed on one above another, till they rose to the height of 36 feet.

By thefe gages it appeared,

ift. That the Sap began visibly to rife MARCH 10, when the thermometer by day flood only at

3 degrees above the freezing point;

2dly. That, APRIL 18, it was at its height and vigour;

3dly. That from that time to MAY 5th the force gradually decreased;

4thly. That it conftantly rose fastest from fun-rife to about 9 or 10 in the morning, and then gradu.

ally fubfided till about 5 or 6 o'clock in the af

ternoon;

5thly. That it rose fooner in the morning after cool weather, than after hot days, and in proportion to the coldness of the night and subsequent heat ; 6thly. That after feveral fucceffive cold days and nights, the fap would rife during the whole day, if it chanced to be fine, although flowest at noon. 7thly. That if warm weather had made the fap flow

vigorously, that vigour would be abated immediately by a cold easterly wind and a cloudy fun,

when the fap would fink at the rate of an inch per minute; but when the fun shone out, and the wind shifted, it rofe again as ufual.

8thly. The OLDEST VINES were foonest affected by a change of temperature, and in them the sap firft began to fink.

9thly. And, on the contrary, when the tube was fixed to a very short stump of a YOUNG VINE, and at only 7 inches from the ground, the fap flowed inceffantly, and fafteft of all, in the greateft heat of the day, finking only after fun-fet*.

He then makes this general conclufion, that the rapidity with which the fap circulates in the vine during SPRING is five times greater than the rapidity with which the blood flows in the arteries of a horse, that it is confiderably flower in the SUMMER than in spring, very languid in AUTUMN, and ceases altogether in the WINTER.

The above experiments clearly demonstrate, that it is

*This last observation is very valuable, and exactly applies to the human frame. If a child or an old man take the strong stimulus of opium or wine, an exhaustion of the irritability of the fibre enfues, as is fhewn in page 367; but if the fame quantity be given to a person in the vigour of health and life, it feems to call forth the irritability of the fibre without exhausting it, and the actions of life become increased, and fleep does not take place before the cuftomary hour. This will be further illustrated page 417 and 418, on the different Effects of the Cold Bath on strong and very weak Constitutions.

not

not from heat and light alone that the fap rifes in the vine, for if that were the case it would increase as the heat increased, it would be greatest in the noon-day and in the height of fummer, and lefs in fpring than in autumn, whereas the reverfe is here fhewn to be the cafe. It must therefore depend on the IRRITABILITY of the fibre, which gets exhausted by the ftimulus of heat and light, and is accumulated by its absence.

In the fame way the IRRITABILITY of the bedyfarum gyrans is exhaufted by the heat of the noon-day fun; and, according to the experiments of FONTANA it is proved that the IRRITABILITY of the fenfitive plant is great in the morning, diminished during the heat of the day, and little or none in the evening.

Hence it is that the return of cold and froft in the Spring is fo noxious to vegetables, and that this feafon is forward according to the feverity of the preceding winter.

FONTANA obferved, that during winter the vipers which he kept for his experiments were in a torpid state, though the thermometer was at 59 degrees. He endeavoured to render them vigorous by warmth, and exposed them to a heat of 67 degrees only. In two minutes they died, though during fummer they bear a much greater :) VOL. II. 3 H

degree

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