190 SPRINGS, RIVERS, CANALS, LAKES?, 



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of acids \vith alumine, magnesia, lime, and metals. When llils af- 

 kaH precipitates a mineral water, it cannot, therefore, be known by 

 simple inspection or the precipitate, of what nature the earthy salt 

 decomposed in the experiment may be. Its effect is still more un- 

 certain, when the alkali made use of is saturated with carbonic 

 acid, as is most commonly the case, since the acid to which it is 

 united, augments the confusion of effects : for this reason, the use 

 of very pure caustic potash is proposed, which likewise possesses an 

 advantage over the effervescent alkali, viz. that of indicating the 

 presence of chalk dissolved in a gaseous water, by virtue of the su- 

 perabundant carbonic acid : for it seizes this acid, and Vhe chalk 

 i'alls down of course. This fact is established by pouring soap lees 

 newly made, into an artificial gaseous water, which holds chalk 

 in solution. The latter substance is precipitated in proportion as 

 the caustic fixed alkali seizes the carbonic acid which held it in solu- 

 tion. By evaporating the filtrated water to dryuess, carbonat of 

 soda is obtained, strongly effervescent with acids. The caustic 

 fixed alkali likewise occasions a precipitate in mineral waters, 

 though they do not contain earthy salts ; for if they contain an alka- 

 line neutral salt, of a less soluble nature, the additional alkali will 

 precipitate it by uniting with the water, nearly in the same manner as 

 alkohol does. M. Gioanctti has observed this phenomenon in the 

 \vaters of St. Vincent ; and it may easily be seen by pouring caustic 

 ^alkali into a solution of sulphat of potash, or muiiat of soda; these 

 two salts being quickly precipitated. 



Caustic ammoniac is in general less productive of error when 

 mixed tvih mineral waters; because it decomposes only salts, with 

 blue of,alumine or magnesia, and does not precipitate the calca- 

 reou? Mfcgv It is necessary, however, to make two observation* 

 respecting this *i!t : the first is, that it must be exceedingly caustic, 

 or totally deprived of carbonic acid ; without this precaution, it 

 decomposes calcareous salts by double affinity : the second is, that 

 the mixture must not be left exposed to the.air, when the effect of 

 its action is required to be inspected several hours after it wadded ; 

 because, as M. Gioanetti has well observed, this salt in a very short 

 time seizes the carbonic acid of the atmosphere, anil becomes ca- 

 pable of decomposing calcareous salts. To put this important fact 

 out of doubt, Fourcroy made three decisive experiments ; some 

 grains of sulpha! of lime, formed of transparent calcareous spar, 



