400 PHILOSOPHICAL TRANSACTIONS. [aNNO I767. 



of rain water, and then mixed with 6^ ounces of lime-water. A precipitate was 

 immediately made on mixing, which could not be re-dissolved on shaking. 



Exper. g. — Mr. C. made some experiments to find whether the unneutralized 

 earth could be precipitated from other London waters, by the addition of lime- 

 water, as well as from Rathbone-place water. It is necessary for this purpose, 

 that the quantity of lime-water should be adjusted very exactly; for, if it is too 

 little, it does not precipitate all the unneutralized earth ; if it is too great, some 

 of the earth in the lime-water remains suspended. For this reason, as he found 

 it almost impossible to adjust the quantity with sufficient exactness, he added such 

 a quantity of lime-water, as he was well assured was more than sufficient to pre- 

 cipitate the whole of the unneutralized earth; and when the precipitate was 

 subsided, decanted off the clear liquor, and exposed it to the open air, till all the 

 lime remaining in the water was precipitated, by attracting fixed air from the 

 atmosphere. The clear liquor was then decanted and evaporated, which is much 

 the most exact way he knows, of seeing whether any unneutralized earth remains 

 suspended in the water. The result of the experiments was as follows: — 200 

 oz. of water, from a pump in Marlborough-street, were mixed with 38 oz. of 

 lime-water. The earth precipitated, weighed 38 grs. The clear liquor, exposed 

 to the air, and evaporated in a silver pan till it was reduced to 6 or 7 oz. deposited 

 no more than 2 or 3 grs. of unneutralized earth. A like quantity of the same 

 pump water, evaporated by itself, without the addition of lime water, deposited 

 about IQ grs. of unneutralized earth. 



200 oz. of water, from a pump in Hanover-square, being mixed with Q7 oz. 

 of lime water, the precipitate weighed QS grs. The clear liquor, treated in the 

 same way as the former, deposited about 2 grs. of earth. 200 oz. of the same 

 water, evaporated by itself, deposited 28 grs. of earth. 



The same quantity of water from a pump in St. Martin's church-yard, being 

 mixed with 82 oz. of lime-water, the precipitate weighed 108 grs. The clear 

 liquor deposited scarcely any unneutralized earth on evaporation. The same 

 quantity of water, evaporated by itself, yielded 45 grs. of unneutralized earth. 



The way by which he found the quantity of unneutralized earth deposited on 

 evaporation, was, after having decanted the clear liquor, and washed the residuum 

 with rain water, to pour a little spirit of salt into the silver pan ; which dissolves 

 all the calcarious earth, but does not corrode the silver. Then, having separated 

 the solution from the insoluble matter, the earth was precipitated by fixed alkali. 

 In this way of finding the quantity of unneutralized earth, care must be taken 

 to add very little more acid than is necessary to dissolve the unneutralized earth, 

 and to use as little water in washing out the solution as possible; for otherwise a 

 good deal of the selenite, which is deposited in the evaporation of most water. 



