CAUSE AND EFFECT IDENTIFIED.

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should be seen as like. The experiments of Priestley upon the red oxide of mercury were the turning point in the rapprochement. These experiments showed that when mercury is burned it becomes heavier, by taking in some substance from the air; which substance could again be driven off, and the metallic mercury reproduced. The act of combustion of the mercury was to all appearance identical with the burning of coal in a fire, while the resulting change on the substance the conversion of the metal into a red powder, might suggest the process of the rusting of iron; the chief point of diversity being the time occupied in the two different operations. Through an intermediate phenomenon like this, the flash of identity might pass between the two extremes. It is now known that these are instances of the same natural action, namely, the combination of the solid material with the gaseous oxygen of the atmosphere.

In the great problem of Inductive Science, namely― to discover the effects of all causes, and the causes of all effects-there is a variety of intellectual operations gone through; the problem puts on many different aspects. But there is constantly manifested the importance of a powerful reach of the identifying intellect. Some discoveries turn upon this exclusively; and no extensive series of discoveries can proceed without it. In truth, the very essence of generalization being the bringing together of remote things through the attraction of sameness, this attractive energy is the right hand of a scientific inquirer. To cite the greatest example that the history of science contains-the discovery of universal gravitation, or the identifying the fall of heavy bodies on the earth with the attraction between the sun and the planets;-this was a pure stroke of similarity, prepared by previous contemplation of the two facts apart. Newton had for years been studying the celestial motions: by the application of the doctrines of the composition and resolution of forces to the planetary movements, he had found that there were two influences at work in the case of each planet; that one of these is in the direction of the sun, and the other

in the direction of the planet's movement at each instant; that the effect of the first, acting alone, would be to draw the body to the sun; and the effect of the second, acting alone, would be to make it fly off at a tangent, or in a straight line through space. By this process of analysis he had reduced the question to a much simpler state; he had in fact prepared the phenomenon of planetary motion for comparison with other movements already understood. The analysis was itself a remarkable effort of intellect; no other man of that time showed the capability of handling the heavenly motions with such daring familiarity-of intruding into their spheres the calculations of terrestrial mechanics. The perception of identity could not be long delayed after such a clearing of the way. Newton had familiarized himself, as the result of this mechanical resolution of the forces at work, with the existence of an attractive force in the sun, which acted on all the bodies of the system; and he had discovered, by a further effort of calculation, that this force varies inversely as the square of the distance. As yet the phenomenon of solar attraction stood solitary in his mind, but it stood out as a remarkably clear and definite conception, so definite and clear that if ever he came to encounter any other phenomenon of the same nature, the two would in all probability flash together in his mind. Such was the preparation on the one side, the shaping of one of the two individual phenomena. Then as to the other member. He had been acquainted with the falling of bodies from his infancy, like everybody else; and the impression that it had made, for a length of time, was as superficial as it had been in the minds of his brethren of mankind. It was to him as to them a phenomenon of sensible weight, hurts, breakage; demanding machinery of support and resistance. This was the view naturally impressed upon his mind, and, in this encumbered condition, an identity with the pure and grand approach of the distant planets towards the sun, while held at a vast distance from the great luminary, was not to be looked for, even in the

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mind of Newton, whose identifying reach was, undoubtedly, of the first order. He had been for a length of time in possession of the prepared idea of solar force, without its ever bringing to his view, for comparison, the familiar fact of a body falling to the earth. It was obviously necessary that some preparatory operation should take place upon this notion likewise; some meditative effort that would partially clear it of the accompaniments of mere smash, breakage, weight, support, &c., and hold it up in its purest form, as a general movement of all free bodies towards the earth's surface, or rather in the direction of the earth's centre. There was needed an analytic or disentangling procedure; an operation very distasteful and repellent to the common mind, and stamping the scientific character upon any intellect that is at home in it. At what moment Newton laid his analytic grasp upon this ancient experience of our race, we may not now be able precisely to determine; the commonly recounted incident of the fall of the apple may have been the culminating point, but the course of his studies of terrestrial mechanics was the essential element. One cannot help supposing, that, when the phenomenon was once taken to task in the way he had already been accustomed to deal with such things, he would very soon eliminate the main fact from all the confusing circumstantials, and see in it an instance of the motion of one body towards another, by virtue of some inherent power in the attracting over the attracted mass. This eliminating generalization would present the case pure and prepared to his mind, as the other had already been by a previous operation; and then came the flash of identification, and with it the sublime discovery that brought heaven down to earth, and made a common force prevail throughout the solar system. Not less to his honour than the discovery itself, was his reserving the announcement, until such time as the proof was rendered complete, by an accurate estimate of the magnitude of the earth, which was a necessary datum in the verifying operation.

This great stretch of identification, perhaps the widest leap that the intellect of man has had the opportunity of achieving, not only illustrates the mental attraction of similarity, but also presents in relief the preparation of the mind for bringing on the flash. We see the necessity there was for a powerful mathematical faculty, to seize the laws of the composition and resolution of forces, and to apply them to the complicated case of elliptic motion; in this application, Newton already made a step beyond any mathematician of the age. We observe, in the next place, the intense hold that the mathematical aspect of the phenomena took on his mind, how he could set aside or conquer all the other aspects so much more imposing in the popular eye, and which had led to quite different hypotheses of the cause of the celestial movements. This characteristic shines remarkably throughout the scientific writings of Newton; however fascinating a phenomenon may be, he has his mind always ready to seize it with the mathematical pincers, to regard it in that view alone. His mode of dealing with the subject of light is an instance no less striking than the one we have been now setting forth. There was in him either an absolute indifference to the popular and poetic aspects of a phenomenon, or a preference for the scientific side, strong enough to set all these aside. The example, thus afforded, of uncompromising adherence to the relations of number and measured force, was probably the most influential result of his genius, at a time when physical science was as yet un-emancipated from the trammels of a half-poetic style of theorizing. The purifying or regenerating of the scientific method was far more owing to the example of Newton than to the rhetorical enforcements of Bacon. The human intellect was braced by dwelling in his atmosphere, and his avatar was the foremost circumstance in impressing a superior stamp upon the thought of the eighteenth century.

Besides these two peculiarities of the Newtonian mind— mathematical power, and exclusive regard to the mathematical and mechanical, in other words, the strictly scientific,

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aspect of the phenomena to be studied-I have indicated a third, which, although not radically distinct from these, deserves separate notice; I mean analytic force, or the tendency to separate the effects that an object has on the senses or the mind, and to concentrate the regard on one particular at a time. Thus we have seen that a falling body produces a very complex impression-a gross and multifarious effectand this total mass of sensation and feeling is the popular notion of the phenomenon. No accurate knowledge can grow out of such aggregates; they are the soil of poetry, not of science. I shall illustrate afterwards the nature of this force, or impulse, that resists the totalizing influence of a complex object, and isolates for study and comparison its individual effects; I note it here as the volitional, or what may be loosely styled the moral, element of the scientific intellect; it stood forth in singular grandeur in the mind of Newton.

REASONING AND SCIENCE IN GENERAL.

34. Not to mention the examples that we have just parted from, many of the instances of Similarity already adduced in the course of our exposition are strictly of the nature of science. I think it right, notwithstanding, to devote a separate head to the operation of the law in the various scientific processes, with a view to elucidating farther both it and them. I shall therefore make the illustration fall under the four divisions of Abstraction, Induction, Deduction, and Analogy.

ABSTRACTION, Classification, Generalization of Notions or Concepts, General Names, Definitions.-These designations all point substantially to the same operation-the identifying a number of different objects on some one common feature, and the seizing and marking that feature as a distinct subject of thought; the identification being a pure effort of Similarity. Thus we identify the different running streams that have come under our observation, in consequence of the sameness that appears prominent in the midst of much diversity; any