Merlet’s method for the detection of this acid is the following:[3] Pulverize the substance for examination, then triturate it to an impalpable powder, and mix it with an equal part of bisulphate of potassa.  Heat the mass gradually in a moderately wide test-tube.  The judicious application of heat must be strictly observed, for if the operator first heats the part of the tube where the assay rests, the whole may be lost on account of the glass being shattered.  The spirit-flame must be first applied to the fore part of the tube, and then made to recede slowly until it fuses the assay.  After the mixture has been for some time kept in a molten state, the lamp must be withdrawn, and the part containing the assay severed with a file.  The fore part of the tube must then be well washed, and afterwards dried with bibulous paper.  Should the fluorine contained in the substance be appreciable, the glass tube, when held up to the light, will be found to have lost its transparency, and to be very rough to the touch.

    [3] Quoted by Plattner.

Great care should be observed not to allow this very corrosive acid to come into contact with the skin, as an ulcer will be the consequence that will be extremely difficult to heal.

When hydrofluoric acid comes in contact with any silicious substance, hydrofluosilicic acid gas is always formed.

(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 quantity, while there is an incrustation around the assay of a grey color, with a metallic lustre.  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 glass 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 arsenic is present, heating upon charcoal will quickly lead to the determination of the one from the other.

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TABULAR STATEMENT OF THE REACTIONS OF MINERALS BEFORE THE BLOWPIPE.