SECTION VIII.

MINERALOGY.

BY SIR HENRY DE LA BECHE, C.B., F.R.S., &c.

A GLANCE at the best treatises on mineralogy, even those wherein the matter is most condensed, is sufficient to show that a profound acquaintance with this science can only be acquired by long-continued study, and by means of a competent knowledge of certain other sciences, the aid of which must be obtained properly to comprehend the internal and external structure and chemical composition of minerals. The naval man may nevertheless accomplish much, more especially respecting the mode of occurrence and probable origin of minerals under certain conditions, and he may also add by his researches to the catalogue of known substances of this class.

When we see a diamond, we consider that we have before us an arrangement of the particles of carbon in the most perfect manner, that is, these particles have been enabled freely to adjust themselves, so that they have finally been aggregated in a definite form. So also a ruby or a sapphire presents us with the particles of alumina (with usually some slight admixture of other substances, such as oxide of iron, silica, &c.)

arranged in a definite and most perfect manner, the conditions having been such that they also could freely adjust themselves; with this difference, however, in the case of alumina, that it is not one of the simple substances which chemists consider carbon to be, but a compound of a metal (aluminium) and of oxygen. The ruby and sapphire are well-known transparent minerals, but it is not necessary that the particles of even a simple substance should be arranged in what, in common language, may be termed a perfect manner to make the mineral transparent. We may take as familiar examples of the contrary crystals of gold, silver, and copper.

As the knowledge of mineralogy advanced, it was discovered that there existed an intimate connexion between the chemical composition and physical structure of minerals when their constituent substances could form those arrangements of their particles known to us as crystals. This led to the view that when minerals possess the same chemical composition, they also always present the same crystalline system.

This is now known not to be strictly true. The same bodies have been found to occur under two different and incompatible forms, and to this the term dimorphism has been applied. Certain substances have also been discovered to replace others, without altering the form of a mineral, and to this the name isomorphism has been given. The known dimorphous bodies are very few, not more than about 10 in 350 crystallized minerals. The substances which are isomorphous being ascertained, no very great difficulty is experienced on this head. M. Dufrénoy has well remarked that "it is not necessary, in order to pre

sent the same composition, that minerals should exactly contain the same weight of their simple constituent substances; it is sufficient that there is an exact relation between the bases and the acids they contain, or between their isomorphous substances.” *

The external geometric forms of minerals were, as far back as 1784, discovered by Bergman and Haüy not always to be those which might be considered fundamental or primary, since many were found capable of being split or divided into other forms, representing the solid arising from the free adjustment of the component particles, the body of the crystal and external form being made up of an aggregation of many primary crystals, or of some modification of a primary crystal. Of the aggregation of primary crystals the common mineral, calcareous spar, affords a familiar example. The fundamental crystal of carbonate of lime is a given rhombohedron, yet the external forms of this mineral are very varied, so much so that the Comte de Bournon was enabled to describe nearly 800 modifications of them. The primary form alone, as external, is much more rare.

The crystallization of carbonate of lime also well illustrates dimorphism. The common kind, as we have

Dufrénoy, Traité de Minéralogie,' tome i. p. 19. This is a most excellent work, and should be in the hands of those who desire an extended knowledge of mineralogy. We may also mention Phillips's Elementary Treatise on Mineralogy,' an English edition by Allan, and an American edition by Alger; Dana's System of Mineralogy' (New York and London); Beudant's Traité Elémentaire de Minéralogie;' Rammelsberg's Handwörterbuch des Chemischen Theils des Mineralogie;' Blum's 'Die Pseudomorphen des Mineralreichs; Rose's Elements der Krystallographie;' Dr. Karsten's System der Mineralogie' (Berlin); and others.

seen, has a fundamental crystal of a given rhombohedral form, but there is another kind, named arragonite, wherein, though the proportions of carbonic acid and lime are precisely the same, the crystals are hexagonal prisms. At one time this very different crystalline structure was attributed to the presence of a small per centage of carbonate of strontia (from 0.7 to 41); but as arragonite has been found solely composed of carbonate of lime, this opinion seems abandoned. It has been lately stated that when carbonate of lime is crystallized from a warm solution it takes the form of arragonite, and when from a cold one that of common calcareous spar. It should be added, that the packing of the particles of arragonite is such that the specific gravity of this mineral is greater than that of common carbonate of lime.

At one time, though crystals of definite forms, constant internal structure, and chemical composition, allowing for isomorphous substitutions, were being obtained in a multitude of chemical processes carried on upon the great scale, as well as in the laboratory, much stress was laid upon distinctions between artificial and natural products. Now, however, that bodies, once only discovered in various positions among rocks, have been formed artificially, sometimes by accident, at others by design, there appears a disposition to look at inorganic matter more generally, however convenient it may be to describe those bodies by themselves which have been found in some natural position.

Among the researches which have tended to break down the barriers once thought to exist between natural and artificial minerals, the recent labours of M. Ebelmen may be mentioned as the most remark

able, since among the minerals produced are some commonly considered as insoluble by our processes and infusible in our furnaces, and some of them moreover belong to the class of gems. M. Ebelmen inferred that, inasmuch as many substances in solution in water crystallize when the water is evaporated, he would obtain certain minerals if he dissolved their elementary substances in some body (in a state of igneous fusion) capable of so doing, and which at a still higher temperature would evaporate and leave the elementary substances to adjust themselves in a crystalline form. Most perfect success attended the labours founded on this view, and in this manner he obtained crystals identical with rubies, spinels of various colours, chrysoberyl, chrysolite, and others. Crystals of emerald were also formed from pounded emerald. The crystals of chrysoberyl were of sufficient size to have their optical properties tried, and were found identical with those of the natural mineral.

To classify the natural substances described under the head of mineralogy, very various methods have been adopted, chiefly, however, divisible into those based upon their external characters or chemical composition. The following is that adopted by M. Dufrénoy in 1845, founded on chemical composition.

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