horizontal axes are equal, there is one axis of double refraction, or one direction in which double refraction is not observable; but in the Rhombic and Anorthic Systems, in which the horizontal axes are unequal, there are two axes of double refraction.

2. Polarization has the same relation to crystalline form as double refraction, and is displayed by many minerals, of which Tourmaline is a well-known example.

3. Dichroism is when crystals present different colours when viewed by transmitted light in two different directions, of which examples are afforded by Iolite and Mica. Pleochroism is when the above property is exhibited in more than two directions.

Phosphorescence.

When minerals appear more or less luminous they are said to be phosphorescent. That property may be produced either, 1st, by friction, as in Quartz; 2nd, by heat, as in Fluor Spar; 3rd, by electricity, as in Diamond, Calc Spar, Apatite, and some other kinds of Fluor Spar; and 4th, as in the case of some Diamonds, by exposure to the light of the sun.

Fluorescence.

This name has been given to the peculiar phenomenon exhibited by Fluor Spar, of transmitting one colour and reflecting another (according to Sir J. Herschel) from a stratum of small but finite thickness, adjacent to the surface by which the light enters.

After passing through this stratum, the incident light, though not sensibly enfeebled or coloured, has lost the power of producing the same effect, and therefore may be considered as in some way or other qualitatively different from the original light.

This dispersion of the rays, which takes place near the surface, has been called, by Professor Stokes, Fluorescence. It is exhibited by Green and Yellow Uranite and by Chalcolite; as well as by certain specimens of Apatite, Aragonite, Chrysoberyl, Kyanite and Topaz, but in these latter cases (as in Fluor Spar) the phenomenon is due to the presence of some substance accidentally present in small quantity.*

Streak.

This is a test of considerable importance, as the colour of the powder of a mineral is more constant and to be depended on than the colour of the mineral itself, which is liable to be altered by the accidental admixture of foreign substances. The streak is produced either by scratching the mineral or by drawing it across a piece of white unglazed porcelain; and observing the colour of the powder or of the trace it leaves behind.

Stain.

This character consists in leaving a mark on paper or linen, and is confined to a few soft minerals. Graphite may be distinguished from sulphide of Molybdenum, which it much resembles in other respects, by the mark which it leaves behind when drawn across paper.

INTRODUCTION.

Frangibility, or Tenacity.

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The following terms are employed to denote the relative degrees of tenacity in minerals.

1. Brittle: when the parts of a mineral separate in grains or powder on attempting to cut it with a knife, as in Calc Spar.

2. Sectile. This character is intermediate between brittle and malleable, and is used to denote when pieces may be cut off with a knife without falling to powder, although the mineral, nevertheless, admits of being pulverized under the hammer.

3. Malleable: when slices may be cut off and then flattened out under the blows of a hammer, as is the case with native Gold and Native Silver.

4. Flexible: when the mineral admits of being bent without breaking, and retains the position given to it, as in Talc.

5. Elastic; when, after being bent, the mineral flies back to its original position on the removal of the force, as in Mica.

Fracture.

Minerals are said to possess three kinds of fracture, viz. :—

1. Conchoidal, or Shelly: when the fractured surface displays curved concavities bearing more or less resemblance to those in the inside of a bivalve shell. Flint and glass are good examples of this kind of fracture.

2. Even when the fractured surface is not rendered rough by the presence of any minute elevations or depressions.

3. Hackly when the elevations are sharp or jagged, as in broken iron. The Cleavage of a mineral is altogether distinct from the Fracture, with which it must, in no manner, be confounded. Cleavage denotes that a mineral can be cleaved or divided mechanically in certain directions, yielding smooth surfaces of fracture (called the cleavage-planes), parallel with the faces or planes of the primary crystal.

This may be effected by placing a knife or other sharp edge in a direction parallel with the natural joints, and then giving it a smart blow with a hammer. The minerals which yield to cleavage in one direction only, are said to have a lamellar structure,

Hardness.

The manner of testing the hardness of a mineral is by scratching it with one of those named in the following list; or (which is preferable) by trying each with a file, passing it three or four times, with a rather heavy pressure, over the mineral.

The following scale of hardness, by Mohs, is that generally adopted :—

1. Talc the common laminated green variety.

2. Gypsum: a crystallized variety.

3. Culc Spar: a transparent variety.

4. Fluor Spar: a crystalline variety.

5. Apatite: a transparent variety.

6. Felspar (Orthoclase): white cleavable variety.

This quality (which expresses the resistance which a body offers to being broken or torn) must not be considered identical with Hardness. Some soft minerals may be tough, such as sulphate of lime; others, as Flint, though hard, may be easily broken ; while others, of which Jade is an instance, are at the same time both hard and tough.

Specific Gravity.

The specific gravity of a mineral is a test of very great importance in the identification of minerals, and in some cases (as in those of polished gems for instance) it is almost the only one which can be had recourse to without occasioning injury to the specimen. In such cases the test of hardness does not admit of being applied, and, for the same reason, chemical analysis is out of the question. When, therefore, the test of colour cannot be relied upon, the determination of the specific gravity will almost always solve the difficulty. The mistakes that have been, and are constantly being, made by not determining the specific gravity of polished stones (even by those whose business it is to buy and sell such articles) are remarkable. It will be seen in the body of the work that colourless Jargoons are often sold in the East, and even in Europe, for inferior Diamonds, and similar substitutions are frequently made by dealers and jewellers in this country, not from any wilful intention to deceive, but in consequence of their relying solely on colour, lustre, and general appearance in the identification of gems.

These mistakes might generally be avoided by ascertaining the specific gravity. The process is very simple, all that is required being an accurately adjusted balance, and care in the use of it.

The determination of the specific gravity is effected by first weighing the mineral in the usual manner, and then, in water, suspended by a fine thread or horsehair. As the mineral will be buoyed up by the water in a degree proportionate to the surface it presents, its weight in water will be less than in air, and the difference between the weight in water and the weight in air, or the loss in weight it has sustained by immersion, will represent the weight of a quantity of water equal in bulk to the substance operated on. Now, as the specific gravity of a body is the proportion which its weight bears to that of an equal bulk of water, the weight in air divided by the loss of weight (or the difference of the weight obtained in and out of water) which it has sustained in water, will give the desired relation and be the required specific gravity.

Taste.

This test is, of course, only applicable in the case of minerals which are soluble in water. It is of seven kinds, viz. :—

1. Astringent: as in Sulphate of Iron.

2. Sweetish astringent: as in Alum,

INTRODUCTION.

3. Saline as in Common Salt.
4. Alkaline: as in Soda.

5. Cooling: as in Saltpetre.
6. Bitter: as in Epsom Salts.
7. Sour: as in Sulphuric Acid.

Odour.

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The odours of minerals may be tested by breathing strongly upon them or by friction. They are of six kinds, as follows, viz. :—

1. Alliaceous, like garlic. Arsenical Iron emits this odour by friction. It may be obtained by heat from all the arsenical ores or salts, and is a sure indication of the presence of arsenic in the substance from which it is evolved.

2. Horse-radish odour. The odour of decaying horse-radish is very perceptible on heating the ores of Selenium.

3. Sulphureous. Sulphureous odours are given off by Pyrites when it is rubbed, and by many sulphides when heated.

4. Bituminous, or the odour of Bitumen.

5. Fetid. The odour of sulphuretted hydrogen or rotten eggs is elicited by friction from Quartz and some kinds of Limestone, Anthraconite, &c. 6. Argillaceous. The smell of moistened clay may be detected in Serpentine, Clayslate, and some other minerals, by breathing strongly upon them; and from some, as Pyrargillite, it may be elicited by heat.

Adhesion to the tongue

Is in some cases a useful character, dependent on the capacity of the mineral to imbibe moisture. Lithomarge adheres strongly to the tongue, and is a good example of this character, which is also generally sufficient for distinguishing argillaceous from pure limestones..

Cold.

The cold feel caused by some minerals when taken into the naked hand. Thus various kinds of Rock Crystal and gems may be distinguished from glass, which may be made to imitate them closely, by their relative coolness.

Electricity.

This property may be produced in certain minerals by friction or by heat, the latter being called Pyro-electricity. Tourmaline, Calamine, and Boracite are examples of pyro-electric minerals, as are also Topaz, Axinite, Scolecite, Prehnite, Electric Calamine, Sphene, Rhodizite, Rock Crystal, and Barytes.

Magnetism.

The property of attracting the magnetic needle is most strongly exhibited by Iron and some of its compounds; but Nickel, Cobalt, Platinum, Titanium and Palladium, have also been proved, by the experiments of Faraday, to be magnetic in the sense of iron.

The following is a list of metals arranged in the order of their magnetic powers, as approximatively determined by Faraday:-Iron, Nickel, Cobalt, Manganese, Chromium, Cerium, Titanium, Palladium, Platinum, Osmium— to which may be added Aluminium.

Chemical Composition.

Mineralogy is daily becoming a science more and more based on Chemistry; as it is only by means of the chemical analysis of minerals that we can arrive at a true knowledge of their composition;-that is to say, of the simple substances of which they are composed, and of the manner in which those substances are combined.

It is quite beyond the scope of a work of this kind to do more than allude to the subject, except so far as to point out that the blowpipe offers a simple and ready means of testing minerals, and of determining the species to which they belong.

For the way of using this useful little instrument, the student may consult, with advantage, several treatises. A brief, but extremely clear and wellwritten notice of the mode of using the blowpipe, by Mr. Warington Smyth, Professor of Mineralogy and Mining in the Government School of Mines, is contained at p. 259 of the Manual of Scientific Inquiry, published by the Lords of the Admiralty. This notice is drawn up in so comprehensive and masterly a manner that it has been introduced here (by the kind permission of Mr. Smyth, who has allowed it to be made use of).

"The ordinary blowpipe is so well known as scarcely to need description. Various forms have been recommended by their inventors, but for common

B

purposes it is only important that the orifice be not too large, and that the tube be provided with a reservoir for the reception of the moisture which is carried into it with the breath. The flame of a neatly-trimmed lamp is, undoubtedly, the most convenient, but that of a common candle is quite applicable to the qualitative tests with which we shall have occasion to deal. "In looking at the flame of a candle, we may observe two principal divisions which it is necessary by the assistance of the blowpipe to use separately, since their action on the same substances is so different, as, on the one hand, greatly to facilitate certain processes of analysis, and, on the other, to cause much perplexity unless clearly understood.