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by impulse, or by some other means unknown to me. I use that word here to signify in general any force by which bodies tend towards one another, whatever be the cause. For we must learn from the phenomena of nature what bodies attract one another, and what are the laws and properties of the attraction, before we inquire the cause by which the attraction is performed."'
How immense and fertile the region of inquiry which was thus laid open to the world, I need scarcely hint to this assembly. Yet who can be entirely silent on so magnificent a theme? The earth, the sun, and all the celestial bodies which attend the sun, attract each other mutually. The minutest particles of each of them, and of all other bodies, conform to the same law: each particle acting proportionally to its quantity of matter directly, and to the squares of its distances from other particles reciprocally. Hence flow a variety, not yet, nor perhaps ever to be, exhausted, of interesting and important propositions. Such, for example, as those which relate to the attraction of spheres, spheroids, and other solids, whether homogeneous or not, upon particles in assigned positions;—the forces which retain the planets in their orbits so as to conform to the Keplerean laws ;—the forces which disturb the elliptic motions of the planets and satellites;—the irregularities of the lunar motions and those of the other secondaries ;—the mutations of the planes of the several orbits ;—the figure of the earth and planets ;—the variations of gravity at different points at their surfaces;—the tides;—the oscillations of the atmosphere ;—the variations of atmospheric pressure at different altitudes ;—the refraction of light while passing through the atmosphere ;—the attractions of mountains ;—the precession of the equinoxes, and the nutation of the earth's axis;—the irregular figure and balancing of Saturn's ring, and the dependence of that balancing upon that irregular figure; —the librarian of the moon:—these and many more topics of investigation springing from the theory of attraction, and each and all of them, as they are pursued, supplying greater or less confirmation of the existence, "the simple and sublime agency, of this commanding principle." By this principle philosophers have, so to speak, decomposed the physical system of the world, reduced it to its single element, and then re-composed it. Viewed in this relation, physical astronomy is, unquestionably, of all the sciences, the most complete, the most sublime, and that in which the human intellect is most elevated. « But that which gives it, above all, an inestimable value, is its
1 Newton's Opt., Query 31.
2 See Laplace, Exposition du Systeme du Monde, liv. iv. eh. 8.
perfect certainty. Other branches of science have changed incessantly, and several must still undergo modifications, several perhaps be abandoned: yet, whatever be the progress of knowledge, whatever the expansion of intellect, the principle of universal gravitation is established irrefragably, and the main deductions from it rest on an immoveable foundation. Hence it is, as Dr. Chalmers has finely remarked, that "when we look on the days" and discoveries " of Newton, we annex a kind of mysterious greatness to him, who, by the pure force of his understanding, rose to such a gigantic elevation above the level of ordinary men—and the kings and warriors of other days sink into insignificance around him— and he, at this moment, stands forth to the public eye, in a prouder array of glory than circles the memory of all men of former generations—and, while all the vulgar grandeur of other days now mouldering in forgetfulness, the achievements of our great physical astronomer are still fresh in the veneration of his countrymen; and they carry him forward on the stream of time with a reputation ever gathering, and the triumphs of a distinction that will never die!"
Before I. quit this subject, I shall, I trust, be pardoned, if I venture upon one more observation. It has been demonstrated by Lagrange and Laplace, that all the planetary inequalities are Periodical, each returning after a certain time, to go through the same series of changes which it had formerly exhibited; the whole system oscillating, as it were, between certain limits which it can never pass. Now this property, so essential to the well-being of the inhabitants of the several planets, requires the concurrence of these four, independent conditions:—that the force shall be inversely as the square of the distance,—that the masses of the revolving bodies be small, compared with that of the central body,—that the eccentricities of the orbits be inconsiderable,—and that the planes of those orbits be originally not much inclined to each other. The irresistible conclusion thus furnished is, that all this is the work of intelligence and design, for a benevolent purpose; the work of a controlling and regulating Power, from whom all the powers of material nature emanate.
I might now pass to Voltaism, and show, from the testimony of Biot,2 Galvani,3 and Haiiy,' that this new branch of science, (a branch which, under the culture of Davy and others, has been
1 See a most splendid eulogy on the distinguishing qualities of Newton as a philosopher, in the second oi Dr. Chalmers's " Discourses on the Christian Religion viewed in connexion with the Modern Astronomy."
1 Biot, Essai sur l'Histoite generate des Sciences, p. 19.
5 Aloysii Galvani, de Viribus Electricit. in motu musculari Commentar. p. 2.'
4 Haiiy, Nat. Phil., vol. ii.
made to contribute largely to our store of chemical facts and doctrines) may be traced to what Bacon terms " the fortune of experiment." But, lest I should far o'erstep the natural bounds of an oral disquisition, I must satisfy myself with this mere allusion, and proceed to my last selection.
In Physiology, as in physics, we have, occasionally, been more indebted to accident, to a happy, though perhaps at first an erroneous thought, than to the profoundest study, or the best conducted train of experiments. One of the most curious, as well as of the most important functions of the animal economy, is that of digestion, the real nature of which was only hit upon in this manner. Considering the comparatively slender texture of the stomach, and the toughness and solidity of the substances it is capable of digesting, it cannot appear surprising that mankind should have indulged in a variety of theories, and run into a variety of errors, in accounting for its mode of action. Empedocles and Hippocrates ascribed the process to a power, possessed by the stomach, of decomposing the food by a rapid putrefaction: Galen, to its decomposing it by a peculiar accumulation of heat, during the time of digestion: and Macbride and Pringle, long afterwards, to the action of fermentation; thus uniting the two causes assigned by the Greek writers. Grew and Santarelli embraced the doctrine of a concoction, by the various juices that are poured into the stomach from the liver, the spleen, and other organs; while Pitcairn, and all the mechanical physiologists, contended that it was accomplished by a process of trituration, produced by an enormous mechanical pressure of the muscular coat of the stomach upon its alimentary contents, which they were fanciful enough to calculate (as I am assured by a learned friend1 who leads me through this labyrinth) to act with a force exceeding 117,000 pounds, assisted at the same time in its gigantic labor, by an equal force, derived from the surrounding muscles.
Reflecting men, however, were still dissatisfied. Each of these theories was found to be encumbered with difficulties of its own, while all of them were alike incompetent to explain the fact for which they were invented. At this moment Cheselden threw out the fortunate hint, that possibly the process of digestion might be accomplished by some peculiar solvent secreted by one or other of the digestive organs; and he directly pitched upon the saliva, or fluid secreted by the salivary glands. The hint was eagerly seized, and elaborately followed up by Haller, Reaumur, Spallanzani, and various others; and, though Cheselden was under a mistake as to the particular fluid, yet a fluid of a most wonderful solvent power was soon detected, secreted from the internal surface of the stomach
1 John Mason Good, Esq. F. R. S.
itself, and its singular properties were satisfactorily ascertained and established.
This extraordinary menstruum, the most active with which we are acquainted in nature, is now well known by the name of gastricjuice, so called from the Greek term for the organ that pours it forth.1 Its apparent simplicity of composition is as remarkable as its digestive power: for, in a pure and healthy state, it is a thin, transparent, and uninflammable fluid, of a weak saline taste, and utterly destitute of smell. Its antiputrescent property is of as extraordinary a nature as its digestive : for it will render perfectly sweet the most offensive and putrid food that gipsies, or hungry dogs can be made to swallow, in about half an hour after such food has been exposed to its action. This gastric juice farther possesses, in an equal degree, both these curious powers of dissolution and restoration to sweetness, as well out of the body as in the stomach itself.
How majestically quiet and easy is the common walk of nature! (employing the term in its usual acceptation as " a name for an effect whose cause is God")—how simple are the means she makes use of!—how wonderful in their operation !—how adequate to the end proposed! Measuring her, as we too often do, by our own imperfect powers, we take it at once for granted, that that which is to us elaborate, must be equally difficult to herself: in tracing her we are apt to overlook the principle of simplicity which is stamped on all her footsteps; we surround ourselves with a world of machinery, and lose our aim by the mere multiplicity of our agencies; till accident, in a lucky hour, suggests to us a new and easier track, on entering which we are equally surprised at our own dulness, and at the wisdom that beams before us.
But it is more than time that I should conclude. We have seen in the principal cases which I have selected, how much has sprung from apparent accident: I shall scarcely expect to be forgiven by this assembly, if I omit to add, that it was only accident in appearance. How many thousand persons had seen an apple fall, before Newton, with respect to whom the observation had been altogether unproductive? Besides the event of the falling apple, there needed the simultaneous operation of various independent causes to render it an epoch in the history of philosophy. It was necessary that it should be observed by a man at leisure to pursue any train of reflection that should thereby be suggested: it was necessary that it should be noticed by a man of research, and that—not as a lawyer,
} Even the stomach itself, when deprived of vitality, has been found acted upon, and, in a manner, digested by it. See John Hunter, on digestion of the stomach after death. Phil. Trans, vol. lxii. Or, J. Hunter's Observations on the Animal GJconomy, in which this paper is reprinted.
VOL. XIII. Pam. NO. XXVI. 2 M
not as a theologian, —not as an anatomist, a botanist, an entomologist, or a chemist, but as a mathematical philosopher. It was farther necessary that the observer should have a certain fund of previous knowledge, and yet, that his mind should not be pre-occupied. Had the falling apple been observed by Newton when he was absorbed in his admirable investigations concerning light and colors, it might no more have led to the theory of universal attraction and the perfection of physical astronomy, than it would in the contemplation of the most illiterate porter that paces the streets of this metropolis. Let us view these matters aright. It is not chance, but previous design, that in this and other similar instances, brings so many independent circumstances into juxtaposition; just as in the case of two travellers,—one passing from London northwards, the other from York southwards—meeting on the way, the accident of their meeting is a necessary consequence of the previous determinations of both to start at a certain time, and to travel by the same road.
The practical inference we may draw from the whole, and which I would strongly urge upon the younger part of my auditory, is simply this:—Do not hastily relinquish a train of thought suggested by a fresh class of circumstances, even though the prospect of utility be very remote. If the poet spake truly that " our thoughts are heard in heaven," may not a philosopher remark with equal truth, that our noblest thoughts are suggested in heaven, and that all genius is a species of inspiration? Among the ancients, when nature was not, as too frequently happens among us, concealed under a thick veil of elucidation, this was unhesitatingly admitted; as by Plato, by Phadrus, and even by Longinus, the most reserved of ancient critics. They distinguished accurately the enthusiasm or inspiration of genius, from the perturbed suggestions of the 6=oXtj7rToi and Phrenetici; but sought the occasion of both ab extra, and not in the imagination itself. In the latter case, they regarded the bark as driven of necessity, wanting cable and anchor to hold her; in the former, as sailing from choice, because the gale is from a favorable quarter, and the voyage desirable. Under another metaphor they viewed the imagination of the poet, and in its kind and degree, that of every man of genius, as a afield in which the Author of Nature produces a set of objects which existed not before; as a region in which new images and combinations arise, like new plants, under auspicious circumstances of culture or climate, according to the settled laws of the Creator. So fruitful is the womb of nature! So true is it, that we are all, more or less, the children of circumstances; and it behoves us, therefore, to avail ourselves to the utmost of the circumstances into which we are thrown. Distinguish, then, sedulously between the dictates