(12.) _Selenium_ (Se).--This element occurs in combination with lead as the selenide, and with copper as the selenide of copper. It exists also combined with cobalt and lead, as the selenide of these metals; also as the selenide of lead and mercury.
The smallest trace of selenium may be detected by igniting a small piece of charcoal in the flame of oxidation, when the peculiar and unmistakable odor of decayed horse-radish will indicate the presence of that element. An orange vapor is eliminated if the selenium be present in any quant.i.ty, while there is an incrustation around the a.s.say of a grey color, with a metallic l.u.s.tre. This incrustation frequently presents a reddish-violet color at its exterior edges, often running into a deep blue. If a substance containing selenium be placed in a gla.s.s tube, closed at one end, and submitted to heat, the selenium is sublimed, with an orange-colored vapor, and with the characteristic odor of that substance. Upon the cool portions of the tube a steel-grey sublimate is deposited, and, beyond that, can be discerned small crystals of selenic acid. If the mineral be the seleniferous lead glance, sulphurous acid gas will be given off, and may be detected by the smell, or by a strip of moistened litmus paper.
If a.r.s.enic is present, heating upon charcoal will quickly lead to the determination of the one from the other.
TABULAR STATEMENT OF THE REACTIONS OF MINERALS BEFORE THE BLOWPIPE.
In PART THIRD of this work, commencing at page 109, the student will find a sufficiently explicit description of the blowpipe reactions of those princ.i.p.al substances that would be likely to come beneath his attention. The following tabular statement of those reactions--which we take from Scheerer and Blanford"s excellent little work upon the blowpipe--will be of great benefit, as a vehicle for consultation, when the want of time--or during the hurry of an examination--precludes the attentive perusal of the more lengthy descriptions in the text.
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, l.u.s.tre, 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 const.i.tuents of them. For instance, should a minute quant.i.ty 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 pota.s.sa, in the flame of oxidation upon platinum foil. The manganate of soda thus formed will color the fused ma.s.s of a bluish-green tint.
Or a slight quant.i.ty of a.r.s.enic 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 pota.s.sa, and slowly heated on the platinum spoon. By this means the metals are oxidized, while the a.r.s.eniate of pota.s.sa is obtained. Then boil the fused ma.s.s in a small quant.i.ty of water in a porcelain vessel till all tho a.r.s.eniate 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 quant.i.ty to drive off the nitric acid, or, at the utmost, to form a bisulphate with the excess of pota.s.sa. When dry, the salt thus obtained is pulverized in an agate mortar, and mixed with about three times its volume of oxalate of pota.s.sa, and a little charcoal powder.
The mixture is introduced into a gla.s.s 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 a.r.s.enious acid is reduced and the metallic a.r.s.enic sublimed, and which re-condenses in the neck of the bulb. If there the a.r.s.enic be so small in quant.i.ty as to exhibit no metallic l.u.s.tre, 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 a.r.s.enic is volatilized, and may be recognized by its garlic odor.
[4] Quoted by Scheerer.
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 ill.u.s.trations 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 a.n.a.lyst), 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.
Mineral. Diamond
Formula. C
Behavior
in gla.s.s-bulb. --
on platinum foil. In fine powder is slowly consumed without residue in a strong oxidizing Flame.
Mineral. Graphite
Formula. C with some iron silica, etc.
Behavior
in gla.s.s-bulb. Generally gives off water.
on platinum foil. Is slowly consumed leaving more or less ash, princ.i.p.ally Fe^{2}O^{3}.
Mineral. Anthracite
Formula. C + x[.H]
Behavior
in gla.s.s-bulb. Evolves water.
on platinum foil. Is slowly consumed with the exception of a small quant.i.ty of ash.
Mineral. Wallsend-coal
Formula. C, H, O, S and ash.
Behavior
in gla.s.s-bulb. Intumesces and gives off water and tarry matters which partly condense in bulb, and leave a porous c.o.ke.
on platinum foil. Takes fire under blowpipe flame, and burns with a smoky flame, depositing much soot and leaving a porous cinder which burns slowly and leaves a small ash.
Mineral. Cannel-coal
Formula. C, H, N, O, S and ash.
Behavior
in gla.s.s-bulb. As the preceding but gives off more tar.
on platinum foil. Similar to the preceding. If held to the lamp-flame, takes fire and burns for some seconds.
Mineral. Brown-coal
Formula. C, H, N, O, S, and ash.