The alkalies and lime-water also precipitate alumina : this is another source of error, in using those bodies as a test of magnesia. Alumina, however, rarely occurs in mineral waters ; if it does, and is precipitated along with the magnesia, it is easily separated from that earth. For this purpose, after washing the precipitate, it must be boiled in a solution of potassa, by which the alumina is dissolved, and the magnesia is left. Or, the preci pitate may be dissolved in strong muriatic acid, and an alkaline carbonate added to the solution; the precipitate which is thrown down by this must be washed and dried at the temperature of about 100. Diluted muria tic acid is then to be poured on it, which will dissolve the maa,.n•set, hut will not act on the alumina.
51. Alumina is detected by the methods just describ ed for disco% ering the presence of magnesia. Succinic acid, or its compound with ammonia, is another test of alumina, as it precipitates this earth, but does not affect magnesia ; the former methods are, however, generally employed.
52. Silica may be discovered by evaporating the water to dryness, and adding muriatic acid to the residue. If silica be present, it will be left undissolved, and will, when mixed with a small quantity of soda, and subject ed to a strong heat, form a glass; if the quantity of soda be greater, the substance formed will be soluble in water, and the solution on the addition of an acid will deposit the silica.
53 Iron is very easily detected in mineral waters. The triple prussiate (firro-cyanate,) of potassa, when added to the solution of a salt of non, gives a blue pre cipitate. This, however, does not take place, if the water contain any uncombined alkali. II this be present, therefore, it must be neutralized, before adding the prus siate. if the iron also be in the state of protoxid, which is seldom the case, the precipitate is white ; it is neces sary, therefore, in sonic instances, before using the prus siate, to add a small quantity of muriatic acid, by which the protoxide will be converted to peroxide, and will give the blue precipitate with the ptussiatc.
Sonic have objected to this test, as the prussiate it self contains iron, and, if not carefully prepared, will give a blue precipitate when added to a mineral water, which contains an uncombined acid, even though iron he not present. This source of fallacy may be obviat ed, by ascertaining if the prussiate gives a blue tinge to diluted rnuriatic acid. If it do, it must be successively crystallized, till it dues not give the blue colour with the acid ; it may then be employed as an accurate test of the presence of iron.
Infusion of nutgalls is another very delicate test of iron. When this is added to a mineral water, containing a salt of iron with the protoxid, the fluid acquires a dark colour. This test is so delicate, that if one grain of the sup ? ate be dissolved in fifteen gallons of water, the so lution will acquire a dark tinge on the addition of the infusion. In this case, however, the blackness does not appear for some days.
The colour produced by nutgalls, and the delicacy of it as a test, are affected by the presence of other sub stances. An earthy carbonate makes the test much more delicate ; thus the infusion will detect a minute quantity of iron when dissolved in common well-water, which contains carbonate of lime, but will not indicate its presence, when dissolved in the same quantity of distilled water. An alkaline carbonate makes the co lour dark violet ; and, if sulphate of lime be present, the precipitate is at first white, and afterwards becomes black 54. Copper may be detected by evaporating the fluid to chyla SS, dissolving the residue in nitric acid, and adding ammonia to the solution. If this metal be pre sent, it will acquire a blue colour.
Of the Methods employed in ascertaining the quantity of the different ingredients in Mineral Waters.
55. To ascertain the quantity of the oxygen, nitrogen, and carbonic acid, which water contains, the method al ready mentioned 14 a,certaining the nature of the elas tic fluids must be followed. (36 ) The quantity of oxygen will he known by the use of the solution of the sulphutetted hydro-sulphuret of potassa, that of the carbonic acid by means of potassa having previously re moved the sulphuretted hydrogen by the addition of nitric acid to the water in the jar. If the elastic fluid contain sulphurous. acid, its quantity will be ascertained by introducing the peroxid of lead ; the amount of the absorption occasioned by the test for each will indicate its proportion, following the usual precautions of bring ing the elastic fluids to the mean temperature and pres sure.
56. The quantity of sulphuretted hydrogen cannot be found out by boiling the fluid, as the whole of the gas is not in this way expelled. The method recommended by Kirwan, is to fill three-fourths of a jar with the wa ter, and invert it in a water trough ; then throw up ni tric oxide as long as red fumes appear. By this the sul phuretted hydrogen is decomposed, and the sulphur is deposited. This must be collected, and dried with a gentle heat. Each grain of it indicates 3 cubic inches of sulphuretted hydrogen.