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 stumbling-block, 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,13 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 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.14