TESTING, in chemistry, embraces a series of processes, the details of which would occupy far more space than the general plan of this work would admit of. Indeed, testing may be regarded as equivalent to qualitative analysis. As a simple illustration of the process of testing, we will assume that the most common of all chemical compounds, a salt, is submitted for examination. The student must pursue some such course as the following: 1. He must examine the dry substance before the blow-pipe, and note whether (a) it is volatile, as are the salts of ammonia and mercury; or (h) fusible, as are the salts of potash and soda; or (c) infusible, as are the salts of zinc, alumina, magnesia, lime, strontia, and baryta; or (d) reducible, as are the salts of silver, tin, lead, bismuth, anti mony, and cadmium; and (e) whether it gives a coloration to the borax bead, and what that color is. 2. Having made his blow-pipe examination, he must bring his substance to a finely-divided state, and dissolve it, if possible, in water, and if it is insoluble in that fluid, even with the aid of heat, in hydrochloric or nitric acid. The solution, whether in water or acid, to which no test or reagent has been applied, is termed by Odling (A Course of Practical Chemistry, 2d ed. 1865) and others the original solution; and to this are added various tests, such as sulphureted hydrogen, hydrosulphate of ammonia, ammonia, nitrate of silver, etc. The most common effect resulting from the addition of a gaseous or liquid reagent is to cause a precipitate or solid deposit of either the base or acid sought for. These precipitates differ in their color, consistency, etc. ; and the student must note not only the color of the precipitate (although this is the most important point), but also whether the deposit is crystalline, gelatinous, dotty, etc. 3. He must then ascertain to which group the base lie is seeking for belongs. There are three great groups of bases; the members of the first group being precipitated from their acid or acidified solutions by sulphureted hydrogen (hydrosulphuric acid); those of the second group not being thrown down by this reagent, but being precipitated from neutral solu tions by hydrosulphate of ammonia (sulphide of ammonium); while those of the third group are not thrown down by either of these reagents. The first group includes tin, .arsenic, antimony, bismuth, mercury, lead, silver, copper, and cadmium; the second, nickel, cobalt, manganese, iron, chromium, aluminium, and zinc; and the third, barium, strontium, calcium, magnesium, potassium, sodium, and ammonia. (See Odling, op.
cit. p. 64). 4. Having ascertained to which base the group belongs, the next point is to identify it. For information ou this point the reader is referred to any of the standard works on qualitative analysis, or on practical chemistry. 5. The base being thus deter mined, it remains to determine the acid, and in searching for it the student will be much assisted by a knowledge of the solubility of the most important classes of salts. Know ing, for example, the insolubility of the sulphates of baryta and strontia, he need not search for sulphuric acid in a soluble salt of one of these earths. On the other hand, a salt insoluble in water is not likely to be a nitrate or chlorate, or acetate or chloride (the only chlorides insoluble in water being chloride of silver and calomel). We cannot enter into the testing for acids further than to observe that the nitrates and chlorates della grate ; the tartrates and citrates char; the carbonates effervesce when acted on by an acid more energetic than carbonic acid; the silicates. borates, and benZoates are precipitated by hydrochloric acid; and the arseniates and chromates react with hydrosulphuric acid. The presence of any particular acid is more or less indicated by its behavior, while still in union with the base, with strong sulphuric acid, which in many cases causes the evo lution of characteristic fumes or vapors; and among the tests especially applicable for the detection of the acids (in acid solutions) arc solutions of nitrate of baryta, nitrate of silver, chloride of calcium, and perchloride of iron. As the above remarks apply merely to the detection of the base and acid contained in a single salt, it will readily be under stood how much the difficulties are increased when there is a mixture of several salts, or where, in place of a metallic oxide, a vegetable base is present, or where we have to deal with a complicated mixture of organic and inorganic substances, as, for example, in the investigation of the contents of the stomach in a case of suspected poisoning.—The fol lowing works on the subject may be referred to: Fresenius's Qualitative Analysis; Noad's Qualitative Analysis; Greville lYilliams's Outlines of Chemical Manipulation; Bowman's Practical Chemistry; and Odling's Practical Chemistry.