« AnteriorContinuar »
the nervous system, but acting on it from a distance through a medium. Fragrance and heat excite sensations by immediate contact with the organization, as do likewise the absence or presence of certain agents; thus, some things acting on the skin, excite the feeling of touch; the circumstance of the absence of oxygen from the lungs induces a feeling of suffocation; and that which causes organic derangement, causes pain; while objects at a distance from the organization act on it and are seen by the medium of light, and heard by the medium of the air.
Through the muscular sense and the sense of touch thoughts are excited of shape, including the length, breadth, and thickness of objects, and also of their nearness and distance. A blind and deaf man can be as conscious of these as one who hears and sees, though the eye and the ear, being organs which are acted on through a medium, are educated by the former mentioned two senses to apprize us of things at a distance. Persons who have been a long time deaf and blind, on the recovery of their hearing, are merely conscious of a sensation of noise in the ears, but soon are made to perceive, by touch and the muscular sense, in what the sound originated, and consequently which way it comes. The blind often arrive at an extraordinary degree of acuteness in judging of the nearness, distance, and source of sounds, by the education of the ear by the other
Persons, who, having been blind from infancy, have had the operation of couching performed, and their sight thus given to them, have, at first, only been conscious of the small picture on the retina, but by moving towards the object exciting the sensation, and touching it, and then moving back from it, have gradually educated the sense of vision, so as to lose the consciousness of the picture, and regard the object, being in time enabled to judge of its distance by sight. The newborn child, for some time, does not appear to be conscious of the external objects, but merely of the pictures on its retina, and it is not till after the eye
been educated that the child seems to regard through it the presence of surrounding things. Touch appears to be the first sensation excited in it, then the muscular sense; and soon the motive feeling of the stomach instigates it to suck the mamme of its mother, or ought that is placed near its mouth. Other senses are then brought into use, and as the infant advances it may be observed experimenting on the sensation of touch and the muscular sense, by feeling with its fingers the mother's vest or face; gradually the eyes seem attracted by these objects, and the child is eventually made conscious that that which it sees is the same as that which is felt. The sensation of a visible object being a flat picture, can possess no thickness, and it is by experience only that we are enabled to judge through the medium of the eye of the thickness of things. Those who have no scientific knowledge of the heavenly bodies, consider the moon, not as a remote globe similar to our earth, but as a flat surface not very distant from us, because it appears to their eyes to be so.
We judge of the distance of a visible object from knowing its real size; but, without this knowledge, we cannot judge by vision of distance. When a balloon is seen to ascend every one beholds its real magnitude, and gradually as it retires in the horizon and appears a mere speck, the people are aware of its being more and more distant. So it is with
every thing else that we see; if the real size be known, the appear. ing less than that, indicates distance, and this knowledge is acquired by experience however intuitive it may appear. The lines in their various inclinations, formed by the meeting of light, shade, and colours, mark the visible shape of things; and it is by a proper arrangement of these, together with the indication of distance by relative size, that the eye can be so deceived by the panoramic view, or even by a good picture. Some panoramas are so excellent, that one, on looking at them, can scarcely believe but that there is a widely extended view stretched out for miles before him, and often feels desirous to go and touch the painting to feel if the canvass be really flat.
When the sense of sight has been educated, it sometimes corrects the errors of touch, as in the following instance. The second and third fingers are so placed on the hand, that in their general position to excite two sensations, such as may be excited by rubbing the fingers against two marbles, or any two small balls, we must use the two balls; but if the middle finger be bent across the third, so as to bring the two sides generally most remote in close proximity with each other, the two sensations that have been usually excited by two things, may be excited by one; and if one ball or the point of the nose be rubbed by the fingers in this position, we are conscious of the same sensations of touch that have usually been excited by two things, and if the eyes were closed might be actually deceived with respect to the number. If this experiment be commonly practised, however, the influence of experience will remove the deceptiveness of the sensations; for the eyes, and the other sensations of touch, will gradually associate the sensations with one object instead of two. When sight deceives us, we appeal to touch, and when it excites an erroneous consciousness it is corrected by carefully touching with other parts, or by sight.
The senses by which we are made to know external things, thus act reciprocally in correcting and educating each other. The various kinds of touch and muscular sensations, excited by an object, render us conscious of its abilities, such as its hardness, smoothness, roundness, &c. We look at it and are made aware that that which we know to be hard, smooth, round, &c., has a certain visible figure and coloured appearance : thus we come to the knowledge of the sensible things around us, and when we are deceived by one of the senses respecting anything, the error is soon removed by the others. A piece of salt may appear so similar to a snow-ball, to the eye of an inattentive or inexperienced person, that he may be deceived with respect to it; but, on referring to touch, the question is at once decided; and after a few such lessons the eye will be able to detect the dissimilarity by itself, fresh distinctions becoming visible by acquaintanceship.
Our first knowledge of external things is gained by the sensations they cause in us, by acting on our organs of sense ;
after this, some of their actions on each other are made known to us by the same means. The sensible actions of things on us and on themselves are all we know of them. Science merely states that certain things produce certain changes in us, and amongst themselves: it consists in the more important facts respecting the different subjects on which it treats, and the child in becoming acquainted with the colour and shape, and the various feelings excited by different things, together with the changes which they produce amongst themselves, is gaining so much scientific knowledge in gleaning together facts respecting its own nature and that of surrounding objects.
Of the guiding excitements—Of the production of knowledge by external
objects-General laws–Of the re-excitement of knowledge-General laws.
All the objects of nature are phenomena of time, space, and causation. The susceptibility to understand the nature of any thing, therefore, is the susceptibility to be made conscious that that thing has size, that it endures, that it has been caused. The knowledge of things includes the knowledge of the relationships they bear to one another; thus, to be conscious of two things is to be aware that in their modes of space, that is, their shapes, they resemble or differ; in their modes of time, they are of longer or shorter durations; in their modes of causation, they have certain abilities, and susceptibilities to the production of change.
The great processes of change are called motions, and by the motions of things new appearances are continually being exhibited. The heavenly bodies with their atmospheres undergo a continual process of change
of locality. The clouds, the mists, the waters, the air, the globe we inhabit, are all in motion. Life itself as has been shewn is but a whirlpool of motions. The animal and vegetable are continually absorbing into, or assimilating with themselves other substances, and as continually radiating from them the matter which was part of themselves. Taking into consideration the quantity of vaporous matter breathed out from the lungs of a robust man, together with that given off from the pores at every point of his skin, it has been calculated that six pounds weight of substance pass away from him in this manner in the course of a day, and this loss is made up from the food he takes and the air he breathes. When the regular motions of life cease, another class of motions commence, called fermentation and putrefaction; in these changes part of the animal substance moves away in minute particles called effluvia, part unites with the surrounding soil. A similar process is observable in water when its regular motions are checked.
For a substance to exhibit motion, experiment shews that it is necessary that an excitement should exist in it. The excitements with which we are acquainted, by which motions are produced, are those of heat, electricity and consciousness. Solids subjected to the excitement of heat exhibit motion, called expansion; they swell till they become fluids; they then swell into vapour or gas. As the excitement of heat abates in the vapour, it condenses into a fluid, and then into a solid: these motions then depend altogether on the state of excitement of the substance.
The minute chemical motions of substances, it appears, depend on electrical excitements, and the adequacy of these excitements to perform the functions of gravitation, offers a simple explanation for the motions of the heavenly bodies.
A familiar instance of the power of electrical excitements to produce motion, may be exhibited by taking a piece of paper, and having warmed it at a fire, let it be laid against a pane of glass in the window, or on a