that this effect is due to a thin film of oxide of iron, which the surface of the iron and protects it.1

forms upon The law of gravity is so simple, and 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 with those 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 mechanics, but points to some common condition existing in the nebulous mass from which our system has 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 partial exception to the empirical law of Bode concerning the distances of the planets, which circumstance may possibly be due to the same disturbance.

Geology is a science in which accidental exceptions are 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 inexplicable. Floods, streams, icebergs, and other casual agents, may lodge remains in places where they would be wholly unexpected. Though such interfering causes have been sometimes wrongly supposed to explain important discoveries, the geologist must bear the possibility of interference in mind.

1 Experimental Researches in Electricity, vol. ii. pp. 240- -245. Murchison's Silurian System, vol. ii. p. 733, &c.

Scarcely more than a century ago it was held that fossils were accidental productions of nature, mere forms into which minerals had been shaped by no peculiar cause. Voltaire appears not to have accepted 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 pilgrims. In archæological investigations the greatest caution is requisite in allowing for secondary burials in ancient tombs and tumuli, for imitations, forgeries, casual coincidences, disturbance by subsequent races or by other archæologists. In common life extraordinary events will happen from time to time, as when a shepherdess in France was astonished at an iron chain falling out of the sky close to her, the fact being that Gay-Lussac had thrown it out of his balloon, which was passing over her head at the time.

Novel and Unexplained Exceptions.

When a law of nature appears to fail because some other law has interfered with its action, two cases may present themselves; the interfering law may be a known one, or it may have been previously undetected. In the first case, which we have sufficiently considered in the preceding section, we have nothing to do but calculate as exactly as possible the amount of interference, and make allowance for it; the apparent failure of the law under examination should then disappear. But in the second case the results may be much more important. A phenomenon which cannot be explained by any known laws may indicate the interference of undiscovered natural forces. The ancients could not help perceiving that the general tendency of bodies downwards failed in the case of the loadstone, nor would the doctrine of essential lightness explain the exception, since the substance drawn upwards by the loadstone is a heavy metal. We now see that there was no breach in the perfect generality of the law of gravity, but that a new form of energy manifested itself in the loadstone for the first time.

Other sciences show us that laws of nature, rigorously true and exact, may be developed by those who are

ignorant of more complex phenomena involved in their application. Newton's comprehension of geometrical optics was sufficient to explain all the ordinary refractions and reflections of light. The simple laws of the bending of rays apply to all rays, whatever the character of the undulations composing them. Newton suspected the existence of other classes of phenomena when he spoke of rays as having sides; but it remained for later experimentalists to show that light is a transverse undulation, like the bending of a rod or cord.

Dalton's atomic theory is doubtless true of all chemical compounds, and the essence of it is that the same compound will always be found to contain the same elements in the same definite proportions. Pure calcium carbonate contains 48 parts by weight of oxygen to 40 of calcium and 12 of carbon. But when careful analyses were made of a great many minerals, this law appeared to fail. What was unquestionably the same mineral, judging by its crystalline form and physical properties, would give varying proportions of its components, and would sometimes contain unusual elements which yet could not be set down as mere impurities. Dolomite, for instance, is a compound of the carbonates of magnesia and lime, but specimens from different places do not exhibit any fixed ratio between the lime and magnesia. Such facts could be reconciled with the laws of Dalton only by supposing the interference of a new law, that of Isomorphism.

It is now established that certain elements are related to each other, so that they can, as it were, step into each other's places without apparently altering the shapes of the crystals which they constitute. The carbonates of iron, calcium, and magnesium, are nearly identical in their crystalline forms, hence they may crystallise together in harmony, producing mixed minerals of considerable complexity, which nevertheless perfectly verify the laws of equivalent proportions. This principle of isomorphism once established, not only explains what was formerly a stumblingblock, but gives valuable aid to chemists in deciding upon the constitution of new salts, since compounds of isomorphous elements which have identical crystalline forms must possess corresponding chemical formulæ.

We may expect that from time to time extraordinary

phenomena will be discovered, and will lead to new views of nature. The recent observation, for instance, that the resistance of a bar of selenium to a current of electricity is affected in an extraordinary degree by rays of light falling upon the selenium, points to a new relation between light and electricity. The allotropic changes which sulphur, selenium, and phosphorus undergo by an alteration in the amount of latent heat which they contain, will probably lead at some future time to important inferences concerning the molecular constitution of solids and liquids. The curious substance ozone has perplexed many chemists, and Andrews and Tait thought that it afforded evidence of the decomposition of oxygen by the electric discharge. The researches of Sir B. C. Brodie negative this notion, and afford evidence of the real constitution of the substance,1 which still, however, remains exceptional in its properties and relations, and affords a hope of important discoveries in chemical theory.

Limiting Exceptions.

We pass to cases where exceptional phenomena are actually irreconcilable with a law of nature previously regarded as true. Error must now be allowed to have been committed, but the error may be more or less extensive. It may happen that a law holding rigorously true of the facts actually under notice had been extended by generalisation to other series of facts then unexamined. Subsequent investigation may show the falsity of this generalisation, and the result must be to limit the law for the future to those objects of which it is really true. The contradiction to our previous opinions is partial and not total.

Newton laid down as a result of experiment that every ray of homogeneous light has a definite refrangibility, which it preserves throughout its course until extinguished. This is one case of the general principle of undulatory movement, which Herschel stated under the title "Principle of Forced Vibrations" (p. 451), and asserted to be absolutely without exception. But Herschel himself described in the Philosophical Transactions for 1845 a curious appearance in a

Philosophical Transactions (1872), vol. clxii. No. 23.

solution of quinine; as viewed by transmitted light the solution appeared colourless, but in certain aspects it exhibited a beautiful celestial blue tint. Curiously enough the colour is seen only in the first portion of liquid which the light enters. Similar phenomena in fluor-spar had been described by Brewster in 1838. Professor Stokes, having minutely investigated the phenomena, discovered that they were more or less present in almost all vegetable infusions, and in a number of mineral substances. He came to the conclusion that this phenomenon, called by him Fluorescence, could only be explained by an alteration in the refrangibility of the rays of light; he asserts that light-rays of very short length of vibration in falling upon certain atoms excite undulations of greater length, in opposition to the principle of forced vibrations. No complete explanation of the mode of change is yet possible, because it depends upon the intimate constitution of the atoms of the substances concerned; but Professor Stokes believes that the principle of forced vibrations is true only so long as the excursions of an atom are very small compared with the magnitude of the complex molecules.1

It is well known that in Calorescence the refrangibility of rays is increased and the wave-length diminished. Rays of obscure heat and low refrangibility may be concentrated so as to heat a solid substance, and make it give out rays belonging to any part of the spectrum, and it seems probable that this effect arises from the impact of distinct but conflicting atoms. Nor is it in light only that we discover limiting exceptions to the law of forced vibrations; for if we notice gentle waves lapping upon the stones at the edge of a lake we shall see that each larger wave in breaking upon a stone gives rise to a series of smaller waves. Thus there is constantly in progress a degradation in the magnitude of water-waves. The principle of forced vibrations seems then to be too generally stated by Herschel, but it must be a difficult question of mechanical theory to discriminate the circumstances in which it does and does not hold true.

We sometimes foresee the possible existence of exceptions yet unknown by experience, and limit the statement of our discoveries accordingly. Extensive inquiries have shown

1 Philosophical Transactions (1852), vol. cxlii. pp. 465, 548, &c.