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IDENTIFICATION OF THINGS ON COMMUNITY OF USE. 491

that are identical to the eye may yet differ to the taste and the smell, as water, alcohol, and white vinegar. We make a class founded on the common peculiarity, and give a designation implying that, and no more. If, however, the taste or smell is the point we are bent on studying, we do not pass from vinegar to water, but to other sour bodies, as the common acids.

25. Without pursuing farther the instances of aggregate impressions on a plurality of senses, let us next advert to the compounds of Sense and Association. Tools, implements, machinery, and all objects of practical utility, make a class that may stand first in exemplifying this aggregation. A knife, for example, is not simply an object of the senses; it is this and something more. Along with the sensation that i it produces on the touch and the sight, there is an associated impression of its use, or of the cutting operation: and we are almost unable to regard it apart from this other circumstance. The appearance of a knife lying on the table is not the whole knife: the appearance of it in the hand while we feel its form and dimensions, coupling sight and touch, is not the whole knife; they are at best but signs or suggestive particulars that revive in the mind, by association, the full notion of the object. Here, therefore, we have a complication of sense and intellect, of impressions made by an actual object, with ideal or associated impressions, arising from previous junctures when we have seen it put to its use. In this association of sensible appearance with use, the last being only occasionally seen in the reality, and therefore for the most part an idea, or a potentiality,—we have abundant room for the exercise of tracing likeness yoked with unlikeness. We may have similarity in form with diversity of use, and similarity of use with diversity of form. A rope suggests other ropes and cords, if we look to the appearance; but looking to the use, it may suggest an iron cable, a wooden prop, an iron girding, a leather band, or bevelled gear. In spite of diversity of appearance, the suggestion turns on what answers a common end. If we are

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very much attracted by sensible appearances, there will be the more difficulty in recalling things that agree only in the use; if, on the other hand, we are profoundly sensitive to the one point of practical efficiency as a tool, the peculiarities not essential to this will be little noticed, and we shall be ever ready to revive past objects corresponding in use to some one present, although diverse in all other circumstances. We become oblivious to the difference between a horse, a steamengine, and a waterfall, when our minds are engrossed with the one circumstance of moving power. The diversity in these had no doubt for a long time the effect of keeping back their first identification; and to obtuse intellects, this identification might have been for ever impossible. A strong concentration of mind upon the single peculiarity of mechanical force, and a degree of indifference to the general aspect of the things themselves, must conspire with the intellectual energy of resuscitation by similars, in order to summon together in the view three structures so different. We can see, by an instance like this, how new adaptations of existing machinery might arise in the mind of a mechanical inventor. When it first occurred to a reflecting mind that moving water had a property identical with human or brute force, namely, the property of setting other masses in motion, overcoming inertia and resistance,-when the sight of the stream suggested through this point of likeness the power of the animal,—a new addition was made to the class of prime movers, and when circumstances permitted, this power could become a substitute for the others. It may seem to the modern understanding, familiar with water wheels and drifting rafts, that the similarity here was an extremely obvious one. But if we put ourselves back into an early state of mind, when running water affected the mind by its brilliancy, its roar, and irregular devastation, we may easily suppose that to identify this with animal muscular energy was by no means an obvious effect. Doubtless when a mind arose, insensible by natural constitution to the superficial aspects of things, and having withal a great stretch of identifying intellect, such a comparison

WATT'S DISCOVERY OF THE STEAM ENGINE.

493

would then be possible. We may pursue the same example one stage further, and come to the discovery of steam power, or the identification of expanding vapour with the previously known sources of mechanical force. To the common eye, for ages, vapour presented itself as clouds in the sky; or as a hissing noise at the spout of a kettle, with the formation of a foggy curling cloud at a few inches' distance. The forcing up of the lid of a kettle may also have been occasionally observed. But how long was it, ere any one was struck with the parallelism of this appearance with a blast of wind, a rush of water, or an exertion of animal muscle? The discordance was too great to be broken through by such a faint and limited amount of likeness. In one mind, however, the identification did take place, and was followed out into its consequences. The likeness had occurred to other minds previously, but not with the same results. Such minds must have been in some way or other distinguished above the millions of mankind; and we are now endeavouring to give the explanation of their superiority. The intellectual character of Watt contained all the elements preparatory to a great stroke of similarity in such a case;a high susceptibility, both by nature and by education, to the mechanical properties of bodies; ample previous knowledge or familiarity; and indifference to the superficial and sensational effects of things. It is not only possible, however, but exceedingly probable, that many men possessed all these accomplishments; they are of a kind not transcending common abilities. They would in some degree attach to a mechanical education almost as a matter of course. That the discovery was not sooner made, supposes that something farther, and not of common occurrence, was necessary; and this additional endowment appears to be the identifying power of Similarity in general; the tendency to detect likeness in the midst of disparity and disguise. This supposition accounts for the fact; and is consistent with the known intellectual character of the inventor of the steamengine.

26. Let us next consider Natural Objects, as seen by the eye of the naturalist, with a view to catalogue and exhaust all their properties and relations. The Mineral, Vegetable, and Animal Kingdoms, as objects of intellectual curiosity and rational explanation, present, in each of their individual specimens, that mixture of the sensible present with the associated absent, above exemplified in the class of tools or machinery. Each mineral, plant, or animal, is a bundle of impressions, of which the whole cannot be made present to the sense at one time; there being a series of actions upon other individuals to be included in the conception, and these usually held together with the assistance of language. The complication thus presented is a degree beyond the preceding group. In Mineral bodies, we have the concurrence of many attributes in each individual, some sensible, others experimental; and it is under the estranging influence of much diversity that all the classes have been formed. Thus, to take the Metals. Some of these have a very large extent of sameness, as tin, zinc, silver, and lead; so, there is a close resemblance between gold and copper, between iron and manganese. But when we come to mercury, a striking point of diversity starts forth; namely, the liquid form. The influence of this diversity, leading the mind away to water and liquids of every kind, would prevent the rise of metals to the view, but for the strong effect of the two qualities-lustre and weight or specific gravity, which, acting by themselves, could suggest by similarity only such substances as silver, lead, tin, &c. This concurrence of two striking points of sameness, overpowers the diverting influence of the liquid state, and brings mercury to the mind's eye side by side with the metals. But these bodies have been identified with others in the midst of still greater discordance. When Sir Humphrey Davy suggested that metallic substances are locked up in soda, potash, and lime, the identification in his mind proceeded upon resemblances purely intellectual; that is to say, making no appeal to the senses, but arrived at through indirect signs, and represented to the mind by tech

DAVY'S DISCOVERY OF METALS IN THE ALKALIES.

495 nical symbols. He found a class of bodies that had a close agreement with one another, and were termed salts; he saw that some of these consisted of an acid and the oxide of a metal,-as sulphate of iron, nitrate of silver; others consisted of an acid and a substance called an alkali,-as sulphate of soda, nitrate of potash. Here there were a number of bodies brought together in the mind by general agreement; an oxide of a metal in these bodies suggested by similarity of function an alkaline substance, both having the property of neutral- · izing an acid and forming a salt; it was impossible, therefore, not to class together in one group all substances having this property, which was done before the time of Davy, under the name bases. He, then, by a bold venture, asserted that. this common property of neutralizing acids, and making salts, grows out of a still closer identity of character, namely, a common composition; in other words, that the alkalies are ocides of metals too, and that therefore all the bases contain a metal and oxygen. On putting the suggestion to the proof, it was found to hold good; lustrous metallic substances were actually separated from soda, potash, &c.; and the identity made good to the sense as well as to the reason. But to trace identities of this nature, a highly intellectual conception is required to intervene; salts had to be considered, not as appealing to the touch, the taste, and the sight, but as compounded of ingredients represented to the mind by names, figures, and symbols. Had copperas been known only as it appears in a drysalter's store, no such identifications could have grown out of its comparison with other salts. It behoved to be known as sulphuric acid combined with oxide of iron, or symbolically as S O3 + Fe 0, in order to see an analogy between it and Glauber's salts, similarly represented, SO3 + Soda. The scientific identities proceed on scientific conceptions, that is to say, on artificial ways of expressing, by names, numbers, and symbols, the facts that experiment brings to light. The same research led to a stroke of identification that would have been utterly impossible to the common eye, namely, of hydrogen gas with the metals,—a gas with

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