Generalization, (vol. ii. p. 259). Quicksilver is divergent among metals as regards its melting point, and potassium and sodium as regards their specific gravity. Monstrous productions and variations, whether in the animal or vegetable kingdoms, should probably be assigned to this class of exceptions.

Accidental Exceptions.

The third and largest class of exceptions contains those which arise from the casual interference of extraneous causes. A law may be in operation, and, if so, must be perfectly fulfilled, but, while we conceive that we are examining its results, we may have before us the effects of a totally different cause, possessing no connexion with the subject of our inquiry. The law is not really broken, but at the same time the supposed exception is not illusory. It may be a phenomenon which cannot occur but under the condition of the law in question, yet there has been such subsequent interference and modification of the result, that there is an apparent failure of science. There is, for instance, no subject in which more rigorous and invariable laws have been established than in crystallography. As a general rule, each chemical substance possesses its own definite form, by which it can be infallibly recognised; but the mineralogist has to be on his guard against what are called pseudomorphic crystals. In some circumstances a substance, having perfectly assumed its proper crystalline form, may afterwards undergo chemical change; a new ingredient may be added, a former one removed, or one element may be substituted for another. In carbonate of lime the carbonic acid is sometimes replaced by sulphuric acid, so that we find gypsum in the form of calcite; other cases are known where the change is inverted and calcite is found in the form of gypsum. Mica, talc, steatite, hematite, are other minerals

Sometimes a

subject to these curious transmutations. crystal embedded in a matrix is entirely dissolved away, and subsequently a new kind of mineral is gradually deposited in the cavity as in a mould. Quartz is thus found cast in many forms wholly unnatural to it. A still more perplexing case sometimes occurs. Carbonate of lime is one of the substances capable of assuming two distinct forms of crystallization, in which it bears respectively the names of calcite and arragonite. Now arragonite, while retaining its outward form unchanged, may undergo an internal molecular change into calcite, as indicated by the altered cleavage. Thus we may come across crystals apparently of arragonite, which seem to break all the laws of crystallography, by possessing the cleavage of an entirely different system of crystallization.

Some of the most invariable and certain laws of nature are disguised by interference of unlooked-for causes. While the barometer was yet a new and curious subject of investigation, its theory, as stated by Torricelli and Pascal, seemed to be contradicted by the fact that in a well-constructed instrument the mercury would often stand far above 31 inches in height. Boyle showed that the mercury could be made to rise as much as 75 inches in a perfectly cleansed tube, or about two and a half times as high as could be due to the pressure of the atmosphere. Many absurd theories about the pressure of imaginary fluids were in consequence put forthi, and the subject was involved in much confusion until the adhesive or cohesive force between glass and mercury, when brought into perfect contact, was pointed out as the real interfering cause.

Guy-Lussac, again, observed that the temperature of boiling water was very different in some kinds of vessels

h Discourse to the Royal Society,' 28th May, 1684.

i Robert Hooke's 'Posthumous Works,' p. 365.

from what it was in others. It is only in contact with metallic surfaces or sharply broken edges that the temperature is at all fixed at 212° Fahr. The suspended freezing of liquids is another case where the action of a law of nature appears to be interrupted. Spheroidal ebullition seemed at first sight a most anomalous phenomenon; it was almost incredible that water should not boil in a red-hot vessel, or that ice could actually be produced in a red-hot crucible. These paradoxical results are now fully explained as due to the interposition of a non-conducting film of vapour between the globule of liquid and the sides of the vessel. The feats of conjurors who handle liquid metals are readily accounted for in the same manner. At one time the passive state of steel was regarded as entirely anomalous. It may be assumed as a general law that when two pieces respectively of electro-negative and electro-positive metal are placed in nitric acid, and made to touch each other, the electro-negative metal will undergo rapid solution. But when iron is the electro-negative and platinum the electro-positive, the solution of the iron entirely and abruptly ceases. Faraday ingeniously proved that this effect was due to a thin film of oxide of iron, which forms upon the surface of the iron and protects itk.

The law of gravity is of so simple and general a character, and is apparently so disconnected from the other laws of nature, that it never suffers any disturbance, and is in no way disguised, but by the complication of its own. effects. It is otherwise, however, with those entirely secondary laws of the planetary system, which have only an empirical basis. The fact that all the long known planets and satellites have a similar motion from west to east is not necessitated by any principles of science, but points merely to some common condition existing in the k Experimental Researches in Electricity,' vol. ii. pp. 240-245.

nebulous mass from which our system has doubtless been evolved. The retrograde motions of the satellites of Uranus constituted a distinct breach in this law of uniform direction, which became all the more interesting when the single satellite of Neptune was also found to be retrograde. It now became probable, as Baden Powell well observed, that the anomaly would cease to be singular, and become a case of another law, pointing to some general interference, which has taken place on the bounds of the planetary system. Not only have the satellites suffered from this perturbance, but Uranus is also anomalous in having an axis of rotation lying nearly in the ecliptic; and Neptune constitutes a distinct exception to the empirical law of Bode concerning the distances of the planets, which exceptional circumstance may possibly be due to the same disturbance.

Geology is a science in which accidental exceptions are very likely to occur. Only when we find strata in their original relative positions, can we surely infer that the order of succession is the order of time. But it not uncommonly happens that strata are inverted by the bending and doubling action of extreme pressure. Landslips may carry one body of rock into proximity with an unrelated series, and produce results apparently inexplicable1. Floods, streams, icebergs, and other casual agents, may occasionally lodge remains in places where they would be wholly unexpected.

Though such interfering causes may have been often wrongly supposed to explain important discoveries, the geologist must of course always bear the possibility of interference in mind. Scarcely more than a century ago it was yet held by many persons that fossils were accidental productions of nature, mere forms into which minerals had been shaped by no peculiar cause. Voltaire

1 Murchison's 'Silurian System,' vol. ii. p. 733, &c.

appears not to have been able to accept such an explanation; but fearing that the occurrence of fossil fishes on the Alps would support the Mosaic account of the deluge, he did not hesitate to attribute them to the remains of fishes accidentally brought there by travellers or pilgrims. In archæological investigations the greatest caution is requisite in allowing for secondary burials in ancient tombs and tumuli, for imitations, casual coincidences, disturbance by subsequent races, or even by other archæologists, in fact, for a multitude of interfering circumstances. In common life extraordinary events must happen from time to time, as when a shepherdess in France was astonished at an iron chain falling out of the sky near to her feet, the fact being that Guy-Lussac had thrown it out of his balloon, which was passing over her head unseen at the time.

To this class of accidental exceptions I would refer the innumerable breaches of the rules of inflexion in grammar. These rules would be invariable were it not that the forms derived from distinct roots sometimes get mixed together, that mistaken analogies sometimes occasion confusion, and a variety of such disturbing causes produce irregularity. Philology already presents beautiful instances of the manner in which a comprehensive law may be traced out in a thoroughly scientific manner, in spite of apparently inexplicable exceptions.

Novel and Unexplained Exceptions.

When a law of nature appears to fail because some other law has interfered with its action, two cases may obviously present themselves;-the interfering law may be a known and familiar one, or it may have been previously undetected. In the first case, which we have sufficiently considered in the preceding section, we have