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pump and in the open air. They may also be excited under water, under oil, in a great variety of gases; in short, under circumstances which exclude the presence of any chemical agent from without, to which such changes could reasonably be imputed.

Secondly. The contraction of irritability has been supposed to be a property of the muscular fibres. Properties are generally considered as permanent qualities. Thus, the property of gravitation is continually operating, equally when bodies remain at rest and when it produces motion in them, equally whilst I support this book in my hand, and when I suffer it to fall on the table. If, however, so curious an occasional property could belong to matter, we should naturally expect that it would belong to some peculiar quality, or arrangement of matter. But irritability is connected with matter of different quali

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ties and arrangements. The flesh of animals and that of fish are different in quality; the mucilaginous bladders which float in the sea differ from vegetables; yet all are irritable, or possess this power of occasional contraction. Though in general

find irritability connected with a fibrous structure, yet, if we may trust our senses, it is not so in

every

instance. In the hydatid, where no such structure is apparent even with the aid of lenses, we still have evidence of the irritability of life. If also, as I strongly suspect, the muscular fibres be not continued from one end of the muscle to the other, irritability could not in that case be considered as a property belonging to them, since any breach of continuity would completely frustrate the contraction of the whole muscle.

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Hunter's opinion, that irritability is the effect of some subtile, mobile, invisible substance, superadded to the evident structure of muscles, or other forms of

vege table and animal matter, as magnetism is to iron, and as electricity is to various substances with which it may be connected. Mr. Hunter doubtless thought, and I believe most persons do think, that in magnetic and electric motions, a subtile invisible substance, of a very quickly and powerfully mobile nature, puts in motion other bodies which are evident to the senses, and are of a nature more gross and inert. To be as convinced as I am of the probability of Mr. Hunter's Theory as a cause of irritability, it is, I am aware, necessary to be as convinced as I am that electricity is what I have now supposed it to be, and that it pervades' all nature. To obtain this conviction it is necessary that the facts connected with this

subject should be attentively considered; but for such an examination I have no time; neither would it be considered as suitable to the general design of these lectures.

Whatever notions philosophers may be pleased to form respecting matter in general, it does not appear to me that our physiological opinions can be affected by their decisions. Of the matter which for the most part presents itself to our notice, and is cognizable by the eye and touch, we know that it has a property called by Sir Isaac Newton vis inertia, an indisposition to move unless impelled to motion, and a disposition to continue in motion unless retarded.

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There are

are some philosophers who think, that properties similar", to those which in the aggregate mass: become an

object of our senses, likewise belong to every atom of which it is composed; whilst others, on the contrary, think, that the atoms have very different qualities, and that the vis inertiæ is the property only of the aggregate mass. The matter of animals and vegetables is, however, an aggregate mass; it is as we express it, common matter, it is inert; so that the necessity of supposing the superaddition of some subtile and mobile substance is

apparent.

Taking it for granted that the opinions generally entertained concerning the cause of electrical inotions are true, analogy would induce us to suppose, that similar motions might be produced, by similar causes, in matter organized as it is found to be in the vegetable and animal systems,

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