1839.] 



On Chemical Tests. 



283 



lxxvi. Ferrocyanuret of potassium is also a good test to show mi- 

 nute quantities of iron in water, by the blue precipitate which it occasi- 

 ons; its action is aided by previously adding two or three drops of ni- 

 tric acid to the water; but it is an equivocal test compared with galls. 



lxxvii. The presence of chlorides or muriatic salts is indicated 

 on adding sulphate of silver by a white cloud, insoluble in nitric acid. 



lxxviii. The sulphates, when present in water, afford a white preci- 

 pitate on the addition of nitrate of baryta, which is insoluble in nitric 

 acid. 



lxxix. Lime is recognised by a white cloud on dropping oxalate of 

 ammonia into the water. A portion of the precipitate collected upon 

 leaf-platinum, and heated before the blow-pipe, may be burned into 

 quick lime. 



lxxx. Magnesia is rendered evident by adding carbonate of ammo- 

 nia, which throws down the lime, and subsequently dropping in phos- 

 phate of soda, which, when magnesia is present, carries it down in the 

 form of a granular precipitate of ammoniaco-magnesian phosphate. 



lxxxi. Such are the readiest means of recognising the presence of 

 the various substances that commonly occur, by the action of re-agents 

 or tests ; and, having gained such general information, we next proceed 

 to the analysis of the water, in order to ascertain the relative propor- 

 tions of the gaseous and saline ingredients which it holds dissolved. 



lxxxii. To ascertain the relative proportions of the gaseous con- 

 tents of water with perfect accuracy, is difficult, and rarely necessary | 

 the following method is sufficiently precise in all ordinary cases. Pro- 

 vide a florence flask capable of holding rather more than a measured 

 wine pint, which quantity of the water under examination is to be 

 introduced into it, and a cork carefully fitted to its neck, having 

 a perforation, in which is inserted a glass tube one-eighth of an 

 inch in diameter, rising perpendicularly about eighteen inches, and 

 then bent so as to pass conveniently under the shelf of the tner- 

 curio, pneumatic apparatus. (Where a sufficiency of mercury can- 

 not be procured, warm water may be substituted, if only carbonic 

 acid be present, and it may be absorbed by transferring the jar 

 containing it to a solution of potassa). The flask should be placed 

 over an argand lamp, and heat gradually applied till the water fully 

 boils. The gas evolved is to be collected in the usual way, in a gra- 

 duated jar over quicksilver, and submitted to the following examination. 



lxxiii. Throw up a small quantity of solution of potassa, which, 

 if carbonic acid be present, will absorb it, and the quantify will be shown 

 by the diminution of bulk, 



