ORYCTOGNOSY.
Tins branch of mineralogy, as already explained, makes us acquainted with all the properties and relations of simple minerals. In tre.tting this subject, we shall con sider it in the following order: 1. Describe the character of simple minerals.
2. Explain the system of arrangement.
3. Describe the different species according to their external characters and chemical composition ; and trace their geognostical and geographical distributions; and, lastly, explain their various uses.
1. Characters of Simple Minerals.
The characters of minerals are of different kinds, viz. External, Chemical, Physical, Geognostical, and Geogra phical.
1. External Characters,—are those which we discover by means of our in the aggregation of minerals, and which have no reference to their relation to other bodies, or to chemical investigations.
2. Chemical Characters,—are those which aro afford cd by the complete analysis of the mineral, by trials with the various re-agents, the blowpipe, and the pyrometer.
3. Physical Characters,—are those physical pheno mena which are exhibited by the mutual action of mi nerals and other bodies ; such are the magnetic and electric properties exhibited by some minerals.
4. Geognostical Characters,—are those derived from various geognostic relations of minerals.
5. Geographical Characters,—are derived from the geographical distribution of minerals.
We shall first consider the external characters, and then the others, in the order already mentioned.
External Characters of Minerals.
The external characters of minerals are either generic or specific. The generic characters are certain proper ties of minerals used as characters, without any refer ence to their differences, as colour, lustre, or weight. The differences among these properties form the specific characters, as adamantine lustre, and glassy or vitreous lustre. The generic characters are divided into general and particular. Under the first, are comprehended those that occur in all minerals, whether solid, friable, or fluid : under the second, those which occur only in particular classes of minerals. In the following tabular view, the external characters are arranged nearly in a natural succession, and in the order in which they are employed in the dcsctiptions of minerals.
Colours of Minerals.
1. Colours.—We begin our description of the exter nal characters of minerals with that of colour, as it is the character which first particularly strikes the eye. It exhibits very great variety, and hence its determi nation is often attended with considerable difficulty. Although it is an important and useful character, it was but ill understood before the time of Werner, and it is even at present, by some mineralogists, considered as of little or no value. The older mineralogists had no very accurate nomenclature of colours, and rarely gave any definition of them ; hence it was, that this character, in their systems, did not afford satisfactory descriptions. Some modern mineralogists, particularly those of the French School, use, in then. descriptions, only single, and often unconnected varieties of colour, which is an erro neous practice ; because in describing species we ought to enumerate all the varieties they exhibit, and in a natu ral order, so that we may obtain a distinct conception of the arrangement of these varieties into groups or suites that characterise the species. Werner was early aware of the utility of this character, and, by a careful study of all its appearances and varieties, was enabled to form a system of colours for the discrimination of minerals, in which he established a certain number of fixed or stand ard colours, to which all the others could be referred, defined the varieties, and arranged them according to their resemblance to these standard colours, and placed them in such manner, that the whole colours in the sys tem formed a connected series.
In establishing the fixed or standard colours, he thought he could not do better than adopt those as sim ple colours, which are considered as such in common life ; of these he enumerates eight, which he denomi nates chief or principal colours ; they are white, grey, black, blue, green, yellow, red, and brown. Although several of these colours arc physically compound, yet, for the purposes of the oryctognost, it is convenient to consider them as simple.