220 MINERAL WATERS, WELL WATERS, ETC. 



in the state of free hydrosulphuric acid, or combined 

 with an alkaline metal (sulphide of sodium, potas- 

 sium, &c.). 



The sulphur of the hydrosulphuric acid, and that of 

 the alkaline sulphides are in general determined at the 

 same time by a method depending upon the decora- 

 position of these compounds by free iodine and upon 

 the coloration that the slightest possible trace of iodine 

 in excess communicates to starch. A standard solu- 

 tion of iodine is made containing 1.27 grams of the 

 iodine to a litre. 1 litre of this solution precipitates 

 0.16 gr. of sulphur, consequently, 1 cubic centimetre 

 of it precipitates 0.00016 gr. A definite volume of 

 sulphur water being placed in a flask a small quantity 

 of starch is added to it; by means of a graduated 

 cylinder, Fig. 27, the standard solution of iodine is 

 gradually poured into the water, shaking the flask ; a 

 drop of iodine in excess colors the liquid permanently 

 blue. 



III. The total weight of the fixed constituents is ascer- 

 tained by evaporating a measured quantity of the 

 water to dryness, and carefully heating the residue to 

 about 200. Should the water contain much chloride 

 of magnesium, an error will result from the partial 

 decomposition of that salt, hydrochloric acid and 

 magnesia being produced ; this may, however, be 

 avoided by dissolving a weighed quantity of pure 

 ignited carbonate of soda in the water before eva- 

 porating. 



IV. The carbonates of protoxide of iron, protoxide of 

 manganese, lime and magnesia, held in solution by free 

 carbonic acid, are precipitated when a large quantity 

 of water is boiled for an hour in a flask. The pre- 

 cipitate is filtered off', dissolved in hydrochloric acid, 

 the sesquioxide of iron precipitated by ammonia, and 

 the protoxide of manganese, lime and magnesia sepa- 

 rated as in No. 25. 



