VI. In the next plate may be ranked the instances which Bacon calls analogous, or parallel These consist of facts, between which an analogy or resemblance is visible in some particulars, notwithstanding great diversity in all the rest. Such are the telescope ' and microscope, in the works of art, compared with the eye in the works of nature. This, indeed, is an analogy which goes much beyond the mere exterior ; it extends to the internal structure, and to the principle of action, which is the same in the eye and in the telescope,—to the latent schematism, in the language of Bacon, as far as material . substance is concerned. It was the experiment of the camera Oscura which led to the discovery of the formation of the images of objects in the bottom of the eye by the action of the crystalline lens, and the other humours of which the eye is formed..
Among the instances of conformity, those are the most useful which enable us to com pare productions of an unknown formation, with similar productions of which the forma tion is well understood. Such are basalt, and the other trap rocks, .compared with the lava thrown out from volcanoes. They have a structure so exactly similar, that it is hardly possible to doubt that their origin is the same, and that they are both produced by the action of subterraneous fire. There are, however, amid their similarity, some very remarkable differences in the substances which they contain, the trap rooks con taining 'calcareous 'spar, and the lava never containing any. On the supposition that they 'are both of igneous origin, is there any circumstance, in the conditions in which heat may have been applied to them, which can account for this difference ? Sir James Hall, in a train of most philosophical and happily contrived experiments, has explained the nature of those conditions, and has shown that the presence of calcareous spar, or the want of it, may arise from the greater or less compression under which the fusion of the basalt was performed. This has served to .explain a great difficulty in the history of the mineral kingdom.
Comparative anatomy is full of analogies of this kind, which are most instructive, and useful guides to discovery. It was by remarking in the blood-vessels a contrivance si milar to the valves used in hydraulic engines, for preventing the counter current of a fluid, that Harvey was led to the discovery of the circulation of the blood. The ana logies between natural and artificial productions are always highly deserving of notice.
• The facts of this class, however, unless the analogy be very close, are apt to mislead, by representing accidental regularity as if it were constant. Of this we have an example in the supposed analogy between the colours in the prismatic spectrum and the divisions of the monochord. The colours in the prismatic spectrum do not occupy the same propor tion of it in all cases : the analogy depends on the particular kind of glass, not on any thing that is common to all refraction. The tendency of man to find more order in things than there actually exists, is here to be cautiously watched over.
VII. Monodic, or singular facts, are the next in order. They comprehend the in stances which are particularly distinguished from all those of the genus or species to which they belong. Such' is the sun among the stars, the magnet among stones, mercury among
Metals, boiling fountains among springs,-the elephant among quadrupeds. So also among the planets, Saturn is singular from his ring, the new planets are so likewise from their small size, from being extrazodiacal, &c.
Connected with these are the irregular and deviating instances, in which nature seems to depart from her ordinary course. Earthquakes, extraordinary tempests, years of great scarcity, winters of singular severity, &c. are of this number. All such facts ought to be carefully collected ; and there should be added an account of all monstrous productions, and of every thing remarkable for its novelty and its rareness. Here, however, the most severe criticism must be applied ; every thing connected with superstition is suspicious, as well as whatever relates to alchemy or magic.
A set of facts, which belongs to this class, consists of the instances in which stones have so often of late years been observed to fall from the heavens. Those stones are so unlike other atmospherical productions, and their origin must be so different from that of other minerals, that it is scarcely possible to imagine any thing more anomalous, and more in consistent with the ordinary course of our experience. Yet the existence of this pheno menon is so well authenticated by testimony, and by the evidence arising from certain physical considerations, that no doubt with respect to it can be entertained, and it must therefore be received, as making a part of the natural history of meteors. But as every fact, or class of facts, which is perfectly singular, must be incapable of explanation, and can only be understood when its resemblance to other things has been discovered, so at present we are unable to assign the cause of these phenomena, and have no right to offer any theory of their origin. • VIIL Another class of facts is composed of what Bacon calls instantice comitatus, or Viil Another class of facts is composed of what Bacon calls instantice comitatus, or examples of certain qualities which always accompany one another. Such are flame and heat,—flame being always accompanied by heat, and the same degree •of heat in a given substance being always accompanied with flame. So also heat and expansion,—an increase of heat being accompanied with an increase of volume, except in a very few cases, and in • circumstances very particular.
The most perfect instantia comitatui known, as being without any negative, is that of body and weight. Whatever is impenetrable and inert, is also heavy in a degree propor tional to its inertia. To this there is no exception, though we do not perceive the con nection as necessary.
Hostile instances, or those of perpetual separation, are the reverse of the former.
Examples of this are found in air, and the other elastic fluids, which cannot have a solid form induced on them by any known means, when not combined with other substances. So also in solids, transparency and malleability are never joined, and appear to be incom patible, though it is not obvious for what reason.