In the examination of minerals, before the student avails himself of the aid of the blowpipe, he should not neglect to examine the specimen rigidly in relation to its physical characters, such as its hardness, lustre, color, and peculiar crystallization. It is where the difference of two minerals cannot be distinguished by their physical appearance, that the aid of the blowpipe comes in most significantly as an auxiliary. For instance, the two minerals molybdenite and graphite resemble each other very closely, when examined in regard to their physical appearance, but the blowpipe will quickly discriminate them, for if a small piece of the former mineral be placed in the flame of oxidation, a bright green color will be communicated to the flame beyond it, while in the latter there will be no color. Thus, in a very short time, these two minerals can be distinguished from each other by aid of the blowpipe, while no amount of physical examination could determine that point. The blowpipe is equally an indispensable instrument in the determination of certain minerals which may exist in others as essential or non-essential constituents of them. For instance, should a minute quantity of manganese be present in a mineral, it must be fused with twice its bulk of a mixture of two parts of carbonate of soda, and one part of the nitrate of potassa, in the flame of oxidation upon platinum foil. The manganate of soda thus formed will color the fused mass of a bluish-green tint.
Or a slight quantity of arsenic may be discerned by the following process recommended by Plattner:[[4]] one grain of the finely pulverized metal is mixed with six grains of citrate of potassa, and slowly heated on the platinum spoon. By this means the metals are oxidized, while the arseniate of potassa is obtained. Then boil the fused mass in a small quantity of water in a porcelain vessel till all tho arseniate is dissolved. The metallic oxides are allowed to subside, and the above solution decanted off into another porcelain vessel. A few drops of sulphuric acid are added, and the solution boiled to expel the nitric acid, after which it is evaporated to dryness. In this operation, the sulphuric acid should be added only in sufficient quantity to drive off the nitric acid, or, at the utmost, to form a bisulphate with the excess of potassa. When dry, the salt thus obtained is pulverized in an agate mortar, and mixed with about three times its volume of oxalate of potassa, and a little charcoal powder. The mixture is introduced into a glass bulb having a narrow neck, and gently warmed over a spirit-lamp in order to drive off the moisture, which must be absorbed by a piece of blotting-paper in the neck of the bulb. After a short time, the temperature is increased to a low red heat, at which the arsenious acid is reduced and the metallic arsenic sublimed, and which re-condenses in the neck of the bulb. If there the arsenic be so small in quantity as to exhibit no metallic lustre, the neck of the bulb may be cut off with a file immediately above the sublimate, and the latter exposed to the flame of the blowpipe, when the arsenic is volatilized, and may be recognized by its garlic odor.
If the presence of cadmium is suspected in zinc-blende, it may be detected by fusing a small piece of the blende upon charcoal in carbonate of soda. The peculiar bright yellow sublimate of the oxide of cadmium, if it be present, will not fail to indicate it. This incrustation can be easily distinguished from that of zinc. Thus, with the three illustrations we have given, the student will readily comprehend the great utility of the blowpipe in the examination of minerals.
Although the following tables were not arranged especially for the last part of this work, still this arrangement is so good that by their consultation the student will readily comprehend at a glance what requires some detail to explain, and we feel no hesitation in saying that, although they are not very copious, they will not fail to impart a vast amount of information, if consulted with any degree of carefulness.
The minerals given are such as are best known to English and American mineralogists under the names specified. For more detailed reactions than could be crowded into a table, the student will have to consult the particular substance as treated in Part Third. If this part is perused carefully previous to consulting the tables, these will be found eminently serviceable as a refresher of the memory, and may thus save much time and trouble.
And, finally, we would certainly recommend the student, after he shall have gone through our little volume (if he is ambitious of making himself a thorough blowpipe analyst), to then take up the larger works of Berzelius and Plattner, for our treatise pretends to nothing more than a humble introduction to these more copious and scientific works.