The principal acid of sulphur (the sulphuric, SO^{3}) occurs combined with the earths, the alkalies, and the metallic oxides.  Native sulphur is recognized, when heated upon charcoal, by its odor (sulphurous acid) and the blue color of its flame.  The compounds of sulphur may be detected by several methods.  If the substance is heated in a glass tube, closed at one end, the yellow sublimate of sulphur will subside upon the cool portions of the tube; if the substance should also contain arsenic, the sublimate will present itself as a light brown incrustation, consisting of the sulphide of arsenic.

If the assay is heated in the open glass tube, sulphurous acid will thus be generated; but, if the gas is too little to be detected by the smell, a strip of moistened litmus paper will indicate the presence of the acid.

The assay will give off sulphurous fumes if heated in the flame of oxidation.

If the powdered substance is fused with two parts of soda, and one part of borax, upon charcoal, the sulphide of sodium is formed.  This salt, if moistened and applied to a polished silver surface, will blacken it.  The borax serves no other purpose than to prevent the absorption of the formed sulphide of sodium by the charcoal.  As selenium will blacken silver in the manner above indicated, the presence of this substance should be first ascertained, by heating the assay; when, if it be present, the characteristic horse-radish odor will reveal the fact.

Sulphuric acid may be detected by fusing the substance with two parts of soda, and one part of borax, on charcoal, in the flame of reduction; the mass must now be wetted with water, and placed in contact with a surface of bright silver; when, if sulphuric acid be present, the silver will become blackened.

Or the substance may be fused with silicate of soda in the flame of reduction.  In this case, the soda combines with a portion of the sulphuric acid, which is then reduced to the sulphide, while the bead becomes of an orange or red color, depending upon the amount of the sulphuric acid present.  If the assay should, however, be colored, then the previous treatment should be resorted to.

(6.) Boron, Boracic Acid (BO^{3}).—­This acid occurs in nature in several minerals combined with various bases, such as magnesia, lime, soda, alumina, etc.  Combined with water, this acid exists in nature as the native boracic acid; this acid gives with test paper prepared from Brazil wood, when moistened with water, a characteristic reaction, for the paper becomes completely bleached.  An alcohol solution turns curcuma test paper brown.  Heated on charcoal, it fuses to a clear bead; but, if the sulphate of lime be present, the bead becomes opaque upon cooling.

The following reaction is a certain one:  the substance is pulverized and mixed with a flux of four and a half parts of bisulphate of potassa, and one part of pulverized fluoride of calcium.  The whole is made into a paste with water, and the assay is placed on the platinum wire, and submitted to the point of the blue flame.  While the assay is melting, fluoboric gas is disengaged, which tinges the outer flame green.  If but a small portion of boracic acid is present, the color will be quite evanescent.