Page images
PDF
EPUB

the sun or the north wind, are reckoned by Theo. phrastus to produce the best and purest tar: And the Idæan pines were distinguished from those growing on the plain, as yielding a thinner, sweeter, and better scented tar, all which differences I think I have observed, between the tar that comes from Norway, and that which comes from low and swampy countries.

29. Agreeably to the old observation of the Peripatetics, that heat gathereth homogeneous things and disperseth such as are heterogeneous, we find chemistry is fitted for the analyfis of bodies. But the chemistry of nature is much more perfect than that of human art, inasmuch as it joineth to the power of heat that of the most exquisite mechanism. Those who have examined the structure of trees and plants by microscopes, have discovered an admirable variety of fine capillary tubes and vessels, fitted for several purposes, as the imbibing or attracting of proper nourishment, the distributing thereof through all parts of the vegetable, the discharge of superfluities, the secretion of particular juices. They are found to have ducts answering to the tracheæ in animals, for the conveying of air; they have others answering to lacteals, arteries, and veins. They feed, digest, respire, perspire and generate their kind, and are provided with organs nicely fitted for all those uses.

30. The sap vessels are observed to be fine tubes running up through the trunk from the root. Secretory veslels are found in the bark, buds, leaves, and Aowers. Exhaling vessels for carrying off excrementitious parts, are discovered throughout the whole surface of the vegetable. And (though this point be not so well agreed) doctor Grew in his Anatomy of plants, thinks there appears circulation of the sap, moving downwards in the root, and feeding the trunk upwards.

31. Some difference indeed there is between learned men, concerning the proper use of certain parts of vegetables. But wheiher the discoverers have rightly guessed at all their uses or no, thus much is certain, that there are innumerable fine and curious parts in a vegetable body, and a wonderful fimilitude or analogy between the mechanism of plants and animals. And perhaps some will think it not unreasonable to suppose the mechanism of plants more curious than even that of animals, if we consider not only the several juices secreted by different parts of the same plant, but also, the endless variety of juices drawn and formed out of the same foil, by various species of vegetables ; which must therefore differ in an endless variety, as to the texture of their absorbent vessels and recretory ducts.

32. A body, therefore, either animal or vegetable, may be considered as an organised system of tubes and vessels, containing several sorts of Auids. And as Auids are moved through the vessels of animal bodies, by the systole and diastole of the heart, the alternate expansion and condensation of the air, and the oscillations in the membranes and tunicks of the vesels; even so by means of air expanded and contracted in the tracheæ or vessels made up of elastic fibres, the sap is propelled through the arterial tubes of a plant, and the vegetable juices, as they are rarefied by heat or condensed by cold, will either ascend and evaporate into air, or descend in the form of a grofs liquor.

33. Juices therefore, first purified by straining through the fine pores of the root, are afterwards exalted by the action of the air and the vessels of the plant, but, above all, by the action of the sun's

light;

light; which at the same cime that it heats, doch wonderfully rarefy and raise the fap; till it perspires and forms an atmosphere, like the effluvia of animal bodies. And though the leaves are suppored to perform principally the office of lungs, breathing out excrementitious vapours, and draw. ing in alimentary; yet it seems probable, that the reciprocal actions of repulsion and attraction are performed all over the surface of vegetables, as well as animals. · In which reciprocation, Hippocrates supposeth the manner of nature's acting, for the nourishment and healch of animal bodies, chiefly to consist. And, indeed, what share of a plant's nourishment is drawn through the leaves and bark, from that ambient heterogeneous Auid called air, is not easy to say. It seems very considerable and altogether necessary, as well to vegetable as animal life.

. . 34. It is an opinion received by many, that the fap circulares in plants as the blood in animals: that it ascends through capillary arteries in the trunk, into which are inofculated other vessels of the bark answering to veins, which bring back to the root the remainder of the sap, over and above what had been deposited, during it's ascent by the arterial vessels, and secreted for the several uses of the vegetable throughout all it's parts, ftem, branches, leaves, flowers, and fruit. Others deny this circulation, and affirm that the sap doth noc return through the bark vessels. It is nevertheless agreed by all, that there are ascending and descending juices; while some will have the ascent and descent to be a circulation of the same juices through differenc vessels: others will have the ascending juice to be one fort attracted by the root, and the descending another imbibed hy the leaves, or extremities of the branches : liftly, others think that

che

the same juice, as it is rarefied or condensed by heat or cold, rises and subsides in the fame tube. I shall not take upon me to decide this controversy. Only I cannot help observing, that the vulgar argument from analogy between plants and animals Joseth much of it's force, if it be considered, that the supposed circulating of the fap, from the root or lacteals through the arteries, and thence returning, by inosculations, through the veins or bark vessels to the root or lacteals again, is in no sort conformable or- analogous to the circulation of the blood. .

35. It is sufficient to observe, what all must acknowledge, that a plant or tree is a very nice and complicated machine (a); by the several parts and motions whereof, the crude juices admitted through the absorbent vessels, whether of the root, trunk, or branches, are variously mixed, separated, altered, digested, and exalted in a very wonderful manner. The juice as it passeth in and out, up and down, through tubes of different textures, shapes, and sizes, and is affected by the alternate compression and expansion of elastic vessels, by the viciffitudes of seasons, the changes of weather, and the various action of the solar light, grows still more and more elaborate.

36. There is therefore no chemistry like that of nature, which addeth to the force of fire, the most delicate, various, and artificial percolation (b). The incessant action of the sun upon the elements of -air, earth, and water, and on all sorts of mixed bodies, animal, vegetable and fossil, is supposed to perform all sorts of chemical operations. Whence it should follow, that the air contains all sorts of chenic productions, the vapours, fumes, oils, salts,

[blocks in formation]

and spirits of all the bodies we know : from which general aggregate or mass, those that are proper being drawn in, through the fine vefsels of the leaves, branches, and stem of the tree, undergo in it's various organs, new alterations, secretions, and digestions, till such time as they assume the most elaborate form.

37. Nor is it to be wondered, that the peculiar texture of each plant or tree, co-operating with the - folar fire and pre-existing juices, should so alter the

fine nourishment drawn from earth and air (a), as to produce various specific qualities of great efficacy in medicine : especially if it be considered that in the opinion of learned men, there is an influ. ence on plants derived from the sun, besides it's mere heat. Certainly doctor Grew, that curious anatomist of plants, holds the folar influence to differ from that of a mere culinary fire, otherwise than by being only a more temperate and equal heat.

38. The alimentary juice taken into the lacteals, if I may fo say, of animals or vegetables, consists of oily, aqueous, and saline particles, which being difsolved, volatilised, and diversy agitated, part thereof is spent and exhaled into the air ; and that part which remains is by the æconomy of the plant, and action of the sun, strained, purified, concocted, and ripened into an inspissated oil or balfam, and deposiced in certain cells placed chiefly in the bark, which is thought to answer the panniculus adiposus in animals, defending trees from the wea ther, and, when in sufficient quantity, rendering them evergreen. This balsam, weeping or sweating through the bark, hardens into relin ; and this most copiously in the several species of pines and

[blocks in formation]
« EelmineJätka »