434 On the Nature and ComUnations 



water had become white and turbid , from the suspension of a 

 subtile powder. In several hours after, the cloud had disap- 

 peared, and displayed an abundant and beautiful formation of 

 slender prismatic crystals, which glistened with silvery splendour 

 at the bottom of the vessel. The mass which remained on the 

 filter had become hard, was much diminished, and was very 

 dense. 



1 accounted for the production of crystals by supposing that 

 the precipitate, whatever might be its composition, was insolu- 

 ble in cold water, but solul'.le in verv large portions of boiling 

 water, and hence the crystalline deposition on cooling. Experi- 

 ment, however, showed that the residual hard mass, even when 

 levigated, boiled with water, and filtered, uould no longer afford 

 crvstals on cooling : and tJiis was a sufficient objection to my 

 supposition ; for, if one portion of the compound be soluble, 

 why not the whole ? 



I observed, that even when the precipitate newly obtained 

 was washed with portions of cold water until it no longer altered 

 vegetable colours ; yet the first, second, and sometimes the 

 third washing with boiling water would, after the deposition of 

 the crystals, immediately redden litmus. An opinion now sug- 

 gested itself, whidi the following trials greatly strengthened. 



In order to obtain the acid of the saturnine precipitate, the 

 latter was boiled with dilute sulphuric acid : the liquor became 

 red and transparent, while the sulphate of lead subsided to the 

 bottom. Imagining that I had now obtained tJie acid in a state 

 of suflicient pmity, although coloured with adhering vegetable 

 matter, I thought to separate the sediment by filtration, but was 

 disappointed : for the liquor came through very turbid, notwith- 

 standing that the supernatant liquor had been transparent. It 

 was apparent, therefore, that tlie sul])huric acid being in too 

 small a quantity, had displaced but a part of the vegetable acid, 

 that the latter dissolved the remaining part of its own combina- 

 tion with lead, and deposited it when the solution touched any 

 thing cold, thus producing the turbidness. After some hours, 

 the turbidness in the filtered liquor suljsided, forming a stratum 

 on the bottom of the vessel, over which lay a number of cry- 

 stals. 



The theory of the crystalline formation in the washings of the 

 precipitate now became obvious. It appeared that when the 

 saturnine compound was washed with cold water, no other ef- 

 fect than edulcoration was produced : but that hot water par- 

 tially decomposed ♦ihe mass into a super- and a sub-salt, the for- 

 mer of which being soluble in boiling water, filtered through, 

 but on cooling, deposited crystals of the neutral salt, while free 



acid 



