40 



Methods of estimating accurately 



which there is a white precipitate soluble in potass water, and this, 

 if the quantity of iron be not great, may even be a good test if the 

 iron be previously partially de-oxidized by a stream of sulphuretted 

 hydrogen gas. In general, however, alumina and iron in a solution 

 fall together. Iron gives an ultimately rusty precipitate with an al- 

 kaline carbonate, a black with tannin, and if peroxidized, a blood red 

 colour with sulpho-cyanate of potass. Ferro-cyanate of potass is not a 

 good test, as it often gives a blue with an acid solution, owing to a 

 partial decomposition of its own acid. Potass, in a concentrated so- 

 lution of the water, gives a bright yellow crystalline precipitate with 

 chloride of platina, but no ammoniacal salts must be present, as 

 these yield one exactly similar. Soda is recognised when a little of 

 the residuum of the evaporation is exposed to the blow-pipe flame on 

 a loop of platina wire, by an orange -coloured bright cone of flame 

 proceeding from it. A great excess of potass imparts to this a violet 

 tinge. 



If it be desired to ascertain the quantities of the different com- 

 pound salts contained in any water, the analytical chemist must 

 determine those of their acids and bases, and then by calculation 

 according to equivalents, and the best lights afforded by some che- 

 mical facts known regarding the play of elective affinities in such 

 mixtures, proceed to state the compositions in which they exist. 

 Here, though in regard to the most common substances, he has some 

 pretty sure guides, a good deal must be mere probability. One 

 chemist, for instance, supposes that they are in a state of mutual com- 

 bination, forming only one chemical body, not a mixture of different 

 compounds, but this appears rather unlikely. Another thinks, that 

 they exist in the state of such salts as have the greatest affinity for 

 water, i. e. are most soluble, not according to what others imagine, 

 that the strongest acids are united with the strongest bases. This 

 last seems to be a pretty general and probable opinion, but there are 

 a good many facts tending to subvert it. 



So far as is known, when a weak compound saline solution is 

 concentrated by evaporation a play of affinities takes place, by 

 which, at the new degree of concentration, old substances are decom- 

 posed and new combinations formed. Thus, in evaporating sea 

 water, muriate of lime and sulphate of soda, seem to give sulphate of 



