tom of the eye. Whether this nervous expansion be indispensably necessary to the result or not, it is certain that a pressure upon or affection of the tympanum by the external air, is followed by a new state of the mind, known as the sensation or perception of sound.

24. Varieties of the sensation of sound.

The sensations which we thus become possessed of by the hearing are far more numerous than the words and the forms of speech, having relation to them in different languages, would lead us to suppose. It will help to illustrate this subject if we recur a moment to the sense of TASTE. The remark has somewhere been made to this effect, and probably with much truth, that if a person were to examine five hundred different wines, he would hardly find two of them of precisely the same flavour. The diversity is almost endless, although there is no language which distinguishes each variety of taste by a separate name. It is the same in respect to the sensations of sound. These sensations exhibit the greatest variety, although their differences are too minute to be separately and distinctly represented by language.

These views will appear the less objectionable when it is remembered that sounds differ from each other both in the tone and in the strength of the tone. It is remarked by Dr. Reid, that five hundred variations of tone may be perceived by the ear, also an equal number of variations in the strength of the tone; making, as he expressly informs us, by a combination of the tones and of the degrees of strength, more than twenty thousand simple sounds, differing either in tone or strength.

In a perfect tone, a great many undulations of elastic air are required, which must be of equal duration and extent, and follow each other with perfect regularity. Each undulation is made up of the advance and retreat of innumerable particles, whose motions are all uniform in direction, force, and time. Accordingly, there will be varieties also and shades of difference in the same tone, arising from the position and manner of striking the sonorous body, from the constitution of the elastic medium, and from the state of the organ of hearing.

Different instruments, such as a flute, a violin, and a bass-viol, may all sound the same tone, and yet be easily distinguishable. A considerable number of human voices may sound the same note, and with equal strength, and yet there will be some difference. The same voice, while it maintains the proper distinctions of sound, may yet be varied many ways by sickness or health, youth or age, and other alterations in our bodily condition to which we are incident.

25. Manner in which we learn the place of sounds.

It is a fact particularly worthy of notice in respect to sounds, that we should not know, previous to all experience on the subject, whether a sound came from the right or left, from above or below, from a smaller or greater distance. And this will appear the less surprising when we remember that the undulations of air are always changed from their original direction by the channels and the windings of the ear before they strike the tympanum. Abundant facts confirm this statement.

Some

Dr. Reid mentions, that once, as he was lying in bed, having been put into a fright, he heard his own heart beat. He took it to be some one knocking at the door, and arose and opened the door oftener than once before he discovered that the sound was in his own breast. traveller has related that, when he first heard the roaring of a lion in a desert wilderness, not seeing the animal, he did not know on what side to apprehend danger, as the sound seemed to him to proceed from the ground, and to enclose a circle, of which he and his companions stood in the centre.

It is by custom or experience that we learn to distinguish the place of things, and, in some measure also, their nature, by means of their sound. It is thus that we learn that one noise is in a contiguous room, that another is above our heads, and another is in the street. And what seems to be an evidence of this is, that when we are in a strange place, after all our experience, we very frequently find ourselves mistaken in these respects.

If a man born deaf were suddenly made to hear, he would probably consider his first sensations of sound as

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originating wholly within himself. But in process of time we learn not only to refer the origin of sounds to a position above or below, to the right or left; but to connect each particular sound with a particular external cause, referring one to a bell as its appropriate external cause, another to a flute, another to a trumpet.

CHAPTER V.

THE SENSE OF TOUCH.

26. Of the sense of touch in general and its sensations. WE are next to consider the sense of TOUCH. The principal organ of this sense is the hand, although it is not limited to that part of our frame, but is diffused over the whole body. The hand principally arrests our attention as the organ of this sense, because, being furnished with various articulations, it is easily moveable by the muscles, and can readily adapt itself to the various changes of form in the objects to which it is applied.

The senses, which have hitherto been examined, are more simple and uniform in their results than that of the touch. By the ear we merely possess that sensation which we denominate hearing; we have the knowledge of sounds, and that is all. By the palate we acquire a knowledge of tastes, and by the sense of smelling we become acquainted with the odours of bodies. The knowledge which is directly acquired by all these senses is limited to the qualities which have been mentioned. By the sense of touch, on the contrary, we become acquainted not with one merely, but with a variety of qualities, such as the following, heat and cold, hardness and softness, roughness and smoothness, solidity or resistance, extension, and figure; and, in particular, it gives occasion for the origin of the antecedent and more general notion of externality.

§ 27. Idea of externality suggested in connexion with the touch. If man were possessed of the sense of smell alone, it would be found that the earliest elements of his knowl

edge consisted exclusively in sensations of odours. According, however, as these sensations were agreeable or disagreeable, he would acquire the additional ideas of pleasure and pain. And having experienced pleasure and pain, we may suppose that this would subsequently give rise both to the feelings and the abstract conceptions of desire and aversion. But if he had no other sense, all these feelings would seem to him to be internal, not only in their experience, but their origin; in other words, to be mere emanations from the soul itself; and he would be incapable of referring them to an external cause.—If he were possessed of the sense of hearing alone, the result would be similar; his existence would then seem to consist essentially of sounds, as in the other case it would be made up of odours; nor, indeed, by the aid of merely both these senses combined would he be able to form an idea of externality or outwardness.

But this idea is a most important one; it is the connecting thought which introduces us to an acquaintance with a new form of existence, different from that interior existence which we variously call by the names spirit, mind, or soul. This idea first arises in the mind, although it is not directly addressed to that sense, by means of the touch.

There is no question that the other senses might of themselves furnish a basis of considerable extent for the mental action. By means of their aid alone, such a developement of the mind might take place, that we could perceive, think, compare, abstract, reason, and will. And although, under such circumstances, everything would seem to us to be internal, yet we should probably find the mental action unembarrassed and easy, and a source of pleasure. But after a time we decide to move the limbs in a particular direction, and to press the hand or some other part of the body through some hard and resisting substance. It is when we attempt to do anything of this kind, which calls the sense of touch into action, that we find the wonted series of thoughts disturbed, the desire checked, and the volition counteracted. It is probably at this precise position of the mind, with scarcely the interval of a momentary pause of wonder, that

there arises vividly in the soul a new perception, a new thought, which we call the idea of externality or outness. It is the sense of touch which impinges upon the obstacle that stands in our way; and no other sense admits of this peculiar application. It is thus the means of partially disturbing the previous connexion and tendency of thought, and of giving occasion for the rise of the new idea which is under consideration. And this idea, called into existence under these circumstances, becomes associated with all those notions which we subsequently form of matter. It may be of some importance to add here, that we shall have occasion to refer to this idea again. under the head of Original Suggestion. It is to be remembered, that externality is not a direct object of the touch, as extension and hardness are, but that the tactual sense simply furnishes the occasion on which it is formed.

§ 28. Origin of the notion of extension, and of form or figure.

The idea of EXTENSION has its origin by means of the sense of touch. When the touch is applied to bodies, where in the intermediate parts there is a continuity of the same substance, we necessarily form that notion. It is not, however, to be imagined that Extension, as it exists outwardly, and the corresponding notion in the mind, actually resemble each other. So far from any imitation and copying from one to the other, or resemblance in any way, there is a radical and utter diversity. As to outward, material extension, it is not necessary to attend to it here; our business at present is with the corresponding inward feeling. Nor will it be necessary to delay even upon that; the more we multiply words upon it, the more obscure it becomes. As it is a simple idea, we cannot resolve it into others, and in that way make it clearer by defining it. We must refer in this case, as in others like it, to each one's personal experience. It will be better understood in that way than by any form of words.

The notion of extension is intimately connected with, and may be considered in some sort the foundation of, that of the FORM or figure of bodies.-Dr. Brown somewhere calls the Form of bodies their relation to each other in space. This is thought to afford matter for reflection;