Of the Method of detecting the diyerent Substances contained in Mineral Waters.
The substances employed in detecting the ingredients of mineral waters, are called Tests. By the addition of these to the waters, different appearances are produced, by which we are enabled to judge of the presence of those bodies which they contain. The tests required must be very delicate, as the quantity of matter to be indicated, is often very small. In many cases, it is necessary to evaporate the fluid to a certain extent, be fore some of the substances which are present can be detected.
34. Carbonic acid, in its free state, is easily detected. Water, when it contains this acid, sparkles when poured from one vessel to another. When it is added to an infusion of litmus, the blue is converted to red, and on boiling the fluid, or by exposing it for some time to the air, the original blue is restored. If the water be boiled, and then added to the litmus, no change in the colour takes place.
Water containing carbonic acid gives a white preci pitate when added to lime water, which is soluble in .th•ic and muriatic acids. When also an excess of car bonic acid water is added to the precipitate formed with lime water, the precipitate is dissolved, and by boiling the fluid, the superabundant carbonic acid is expelled, and the carbonate of lime is again deposited.
35. Sulphuretted hydrogen. Water containing this acid reddens litmus, and when the fluid is exposed to the air for some time, or is boiled, the blue is restored. In this respect it resembles carbonic acid, and may therefore be mistaken for it. There is, however, little danger of this, as sulphuretted hydrogen is easily dis tinguished by other tests. Sulphuretted hydrogen is known by the odour which it emits. When water con taining it is added to a solution of a salt of lead, as the acetate, it throws down a black precipitate. A piece of silver when immersed in it acquires a dark colour.
36. To ascertain the nature of any other gaseous fluids besides carbonic acid and sulphuretted hydrogen, a re tort must he filled about two-thirds with the water, and the mouth of it made to terminate under a jar, filled with mercury and inverted in the mercurial trough.
The water is then to he boiled for a quarter of an hour, by which the gaseous fluids will pass over into the jar. The elastic fluids which are given off, may be common air, oxygen, nitrogen, carbonic acid, sulphuretted hy drogen, and sulphurous acid ; the two last, however, never exist in the same water.
37. If sulphuretted hydrogen be present, it must be removed by the addition of nitric acid to the water, which will absorb the elastic fluid.
38. If the gaseous fluid contain sulphurous acid, on the introduction of the peroxide of lead its volume will be diminished.
39. The presence of carbonic acid may be known by admitting a solution of potassa, which will absorb it.
40. Oxygen may be discovered by exposing the resi dual air to the action of the solution of sulphuretted hydro-sulphuret of potassa or of lime, which will absorb the oxygen ; if what remains extinguish the flame of a candle, it is nitrogen.
41. The fixed acids rarely exist in their free state in mineral waters. If they do, the water gives a red colour to litmus, which does not disappear on boiling the fluid.
The acids when in combination are easily detected.
42. Sulphuric acid is detected by the addition of ba ryta, which gives a white precipitate with any fluid con taining a sulphate.
Other tests indicate the presence of sulphuric acid, as nitrate of lead. This, however, is fallacious, as it gives precipitates with other substances besides sulphuric acid.
In using baryta as the test of sulphuric acid, we must be aware that carbonic acid likewise gives a precipitate with baryta ; but this is easily distinguished from that occasioned by sulphuric acid, the latter being insoluble, the former being soluble in muriatic and nitric acids.
Nitrate or muriate of baryta is, in general, preferred to the pure earth, as sulphuric acid is by them more easily separated from the substance with which it is in union, owing to the decomposition of the sulphate being effected by double elective attraction. The most pow-. erful of the barytic salts is the muriate. According to Bergman this indicates the presence of sulphuric acid, though diluted with a million parts of water.