282 



On Chemical Tests. 



[Oct. 



upon a platinum capsule ; weigh it while warm, and the mean obtained 

 from this and the former experiment will give the proportion of dry sa- 

 line ingredients within a trifling error. Thus suppose 1000 parts of the 

 above mentioned water give by, evaporation 114-4 dry residue, then 

 110.6 4 114*4 = 225 -f- 2 = 112.5 = quantity of saline matter in a 

 dry state (salts deprived of water of crystallization) existing in the 

 water. 



Having ascertained the relative quantity of foreign matter in the 

 water, the nature of the substances present is next to he inquired into. 

 The substances which have been found in mineral waters are extremely 

 numerous, those which very ordinarily occur are the following :— Oxy- 

 gen, nitrogen, carbonic acid, sulphuretted hydrogen, carbonate of lime, 

 carbonate of magnesia, carbonate of iron, muriate of magnesia, chloride 

 of sodium, sulphate of magnesia, sulphate of soda, and sulphate of lime. 



lxxi. Oxygen and nitrogen exist in the greater number of spring- 

 waters in the proportions constituting atmospheric air; the proportion 

 of nitrogen is, however, not unfrequently predominant. These gases 

 give no peculiar flavour to the water. 



lxxii. Carbonic acid renders waters sparkling and effervescent: it 

 is detected by occasioning a precipitate in aqueous solution of baryta, 

 which dissolves with effervescence in dilute muriatic acid. 



lxxiii. The presence of sulphuretted hydrogen is known by its 

 odour, by the production of a black precipitate, on dropping into the 

 water a solution of nitrate of silver ; and by the deposition of sulphur, 

 on adding a few drops of nitric acid. 



lxxiv. The carbonates arc dissolved in the water by excess of car- 

 bonic acid, and consequently fall upon its expulsion by boiling. Car- 

 bonate of lime and magnesia are deposited in the form of a white pre- 

 cipitate. Carbonate of iron occasions the separation of a brown powder, 

 and the water is blackened by a few drops of tincture of galls. 



lxxv. Mr. Phillips, in his analysis of Bath-waters, has shown that 

 the delicacy of galls, as a test for iron, is affected by the presence of 

 certa'n salts: if the iron be in the state of protoxide, its detection is fa- 

 cilitated by salts with abase of lime, and by alkalies; if in the state 

 of peroxide, lime prevents the action of the test. This is well shown 

 by dissolving a very minute portion of protosulphate of iron in a glass 

 of distilled water, and adding a drop of tincture of galls, which occa- 

 sions no immediate discoloration; but a drop of lime water, or other al- 

 kali, instantly renders the presence of iron evident;* so that the quan- 

 tity of iron present in a water cannot be correctly judged of by the de- 

 gree of precipitation occasioned in it by tincture of galls. 



