THE REAGENTS.

Those substances which possess the property of acting upon other substances, in such a characteristic manner that they can be recognized, either by their color, or by their effervescence, or by the peculiar precipitation produced, are termed reagents.  The phenomena thus produced is termed reaction.  We use those reagents, or tests, for the purpose of ascertaining the presence or the absence of certain substances, through the peculiar phenomena produced when brought in contact with them.

The number of reagents employed in blowpipe analysis is not great, and therefore we shall here give a brief description of their preparation and use.  It is indispensably necessary that they should be chemically pure, as every admixture of a foreign substance would only produce a false result.  Some of them have a strong affinity for water, or are deliquescent, and consequently absorb it greedily from the air.  These must be kept in glass bottles, with glass stoppers, fitted air-tight by grinding.

A. REAGENTS OF GENERAL USE.

1. Carbonate of Soda.—­(NaO, CO^{2}) Wash the bicarbonate of soda (NaO, 2CO^{2}) upon a filter, with cold water, until the filtrate ceases to give, after neutralization with diluted nitric acid (NO^{5}), a precipitate with nitrate of baryta, (BaO, NO^{5}), or nitrate of silver, (AgO, NO^{5}).  That left upon the filter we make red hot in a platinum, silver, or porcelain dish.  One atom of carbonic acid is expelled, and the residue is carbonate of soda.

A solution of soda must not be changed by the addition of sulphide of ammonium.  And when neutralized with hydrochloric acid, and evaporated to dryness, and again dissolved in water, there must be no residue left.

Carbonate of soda is an excellent agent in reduction, in consequence of its easy fusibility, whereby it causes the close contact of the oxides with the charcoal support, so that the blowpipe flame can reach every part of the substance under examination.

For the decomposition and determination of insoluble substances, particularly the silicates, carbonate of soda is indispensable.  But for the latter purpose, we use with advantage a mixture of ten parts of soda and thirteen parts of dry carbonate of potash, which mixture fuses more easily than the carbonate of soda alone.

2. Hydrate of Baryta (BaO, HO).—­This salt is used sometimes for the detection of alkalies in silicates.  Mix one part of the substance with about four parts of the hydrate of baryta, and expose it to the blowpipe flame.  The hydrate of baryta combines with the silicic acid, and forms the super-basic silicate of baryta, while the oxides become free.  The fused mass must be dissolved in hydrochloric acid, which converts the oxides into chlorides.  Evaporate to dryness, and dissolve the residue in water.  The silicic acid remains insoluble.