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Show 152 NATURAL PLA1r. been left after the other tribPs have performed their annual renovation, is the food of those singular vegetables .; and whenever disease comes upon the vegetable structure, and even when a certain stage of corruption is arrived at in the animal, those crypt. ogamea, or plants of hidden production, fail not to appear, and to perform their functions. Nor is there the' least doubt that those little things, and many of them are probably as momentary in their duration as they are minute in their size. are as faithful to the decree of their kind, and that the mysterious action in them, to which we give the name of vegetable life, is as true to its temperature and its humidity, and as strong against the resistance of merely dead matter, as in the most stately oaks of Eng·land, or in those giant pines which wave their spiry tops in mid-heaven on the western shores of North America. . The tendency of heat is, as has been said, always to separate the particles of substances; but it was already mentioned that all of what we call "the principles of things" admit of a certain play, or have, as it were, an extent to which they can be bent or driven, and yet recover themselves, if that which bent or drove them is withdrawn. A bow is no bad illustration here ; because the elasticity of the bow is an instance of one of those very powers. Now when the skilful archer bends his bow, it pulls the string to a perfectly straight line ; then when he grasps the bow with his left hand, sets the arrow upon the string, holds the string on the fingers of his right hand like hooks, that arm being doubled back into that position in which it can bear the greatest strain without moving, which is when the bent fin .. gers are a very little behind and under the right ear; then if he stretches his left arm with proper skill and rapidity, and so plunges the whole mass of his body and the whole effect of its velocity into the bow, the elasticity of the bow gives way, and "the ACTION OF HEAT. 153 c1oth~yard shaft" is drawn to the head. But if the b.o~ ts." made of a trusty-tree" not a jot of its dasticit. y 1s destroy~d, but the more vigorously it is drawn the more 1t accumulates; and if the bowman slips his fingers at the very instant of his utmost stretc~, the returning bow sends the arrow in perfect stlence through the air :fleeter than an eagle. If, however, the bow were too small for the man he could draw it either till it broke or till its substa'nce ~~re so much injured. that it would not spring; and tf tt were made of bnttle wood, or of a pliant osier twig, it might be overcome by the strength of a child. It is .the same with matter in resisting heat: in ~orne kmd.s of matter. th~re is much resistance, and 1n other kmds there 1s little ; but there is none in which there is not some resistance; and there is perhaps no substance that becomes . sensibly hot to the full extent o~ the h.eat applied to it, but shifts its b.ul~, of course msenstbly, by the very slightest vanatwns of temperature ; when, however, the resistance of the . s~tbstance is overcome, and there is no other oppostt.wn to th~ motion produced by the heat, no m?re sensible he~t 1s shown; though it continues to ~nve off the particles of the substance until if it be 111 the f~ee ~ir, they are dissipated through 'that, and t~e object IS lost to the senses, except indeed th~ VIewless and touchless particles remain to bid ad1~u to the sense of smelling; and it is not a little curwus tha~ that .sense, which has much less apparent connexwn with external things than some of the other senses, .should yet be, in many instances, the first to find thmgs, and the last to lose them. Aft~r t~e power of heat has overcome that of cohesiOn m the heated substance, so as that substance wo.ulJ. spread in vapour through the thin air, the heat mstantly commences its attack upon the yessel, or whatever else confines the matter which lt has overcome1 and subdued to its purpose. The |