CATARACTS, AND INUNDATIONS 1*97 



than half a grain of muriat of soda in the pint, ascertains the 

 presence of the muriatic acid with great certainty and facility ; but 

 they do not in the same manner indicate the presence of the sul- 

 phuric acid, since, according to Bergman's estimate, at least thirty 

 grains of sulphat of soda must exist in the pint of water, in order 

 to produce an immediate sensible effect. To this we may add, that 

 fixed alkali, chalk, and magnesia, precipitate the nitric solution of 

 silver in a much more evident manner, and consequently that the 

 precipitation formed in a mineral water by this solution is insufficient 

 to determine with precision, tire saline or earthy substances from 

 which it arose. 



The solution of mercury by the nitric acid, is still more produc- 

 tive of error : it not only indicates the presence of the sulphuric 

 and muriatic acids in waters, but it is likewise precipitated by the 

 earthy and alkaline carbonats, in a yellowish powder, which might 

 be mistaken for an effect of the sulphuric acid. It has been com- 

 monly supposed, that the very abundant white precipitate which it 

 forms in water, is owing to the presence of a muriatic salt ; yet 

 mucilaginous and extractive substances exhibit the same phenome- 

 non, as is now well known to all chemists. Besides, these sources 

 of error and uncertainty, dependent on the property which several 

 substances have, of producing similar precipitates with the nitric so- 

 lution or' mercury, there are likewise others which depend on the 

 state of this solution itself, and which it is of the utmost conse- 

 quence to know, in order to avoid very considerable errors in the 

 analysis of waters. Bergman has mentioned some of the remark- 

 able differences observed in this solution, according to the manner 

 hi which it is made, either with or without heat, more particularly 

 with respect to the colour of the precipitates it affords by different 

 intermediums ; but he does not say a word concerning the property 

 this solution possesses, of being precipitated by distilled water, when 

 it is highly charged with tiie oxyd of mercury ; though Monnet 

 mentions this fact in his treatise on the dissolution of metals. As 

 this subject is of great importance in the analysis of waters, Four 

 croy endeavoured by a very minute investigation to arrive at some 

 degree of certainty concerning it, and has succeeded, as shall pre- 

 sently appear by very simple means. He has made a great number 

 of solutions of mercury, in very pure nitric acid, with different 

 jsioses of these two substances, with heat and in the cold, and with 



03 



