OS" THE PROCESSES *QR CLARIFYING LIQUIDS. £>55 



poftefs the fame fluidity. Thefe falts confequently become 

 capable of palling through the fineft filters. No other means 

 but evaporation is therefore adopted to feparate water from 

 ihe faline fubftanees which it holds in folution, and conse- 

 quently dlftUIation alone can produce this deiirable eftecl. 

 But to return to the general effects of filtration : 



Independently of the filters here mentioned, water is alio The filtering 

 clarified by means of the ftones called filtering ftones. Of thefe ohe » 

 there are feverai kinds. They are very porous, becaufe fand 

 enters into the greateft part of their compoiition. Thefe ftones 

 being excavated are filled with water. The fluid gradually 

 iniinuates itfelf through their pores, and appears on the ex- 

 ternal furfnee in the form of drops of conflderable clearness, 

 Byhich fall into a veflfel placed beneath. 



It is neceflary that thefe ftones fhould be previoufly warned how managed. 

 jby filtration of a large quantity of water. It is obferved, that • 

 even for feverai days the water acquires a djfagreeable tafte, 

 owing to the foreign masters diflblved by this fluid in its paf- 

 <age through the ftone. Water thus clarified cannot be fit 

 for ufe until it paftes through intirely deprived of tafte. 



In general the filtering ftone, though highly extolled, is a it is a bad con- 

 bad inftrument to procure good water ; for the filtration is tnvanc e« 

 made very flowly, and very often ftops altogether, if the in- 

 ner and exterior furfaces of the ftone be not rubbed from time 

 to time with a coarfe brufh, to detach the earth which the 

 water depofjts. . To thefe inconveniences no doubt it is, that 

 we ought principally to attribute the difufe into which this 

 kind of filtration is fallen. 



It now only remains, that we fhould fpeak of the other Clarifying with- 

 procelfes which are ufed to give that perfect limpidity to fe- out the fiIter * 

 veral fluids, which they can never acquire by fpontaneous 

 clarification or by filters, whatever may be their ftructure. 



Though it is true, that ihe remarkable opacity of fome Opacity from 

 fluids is owing to the interpofition of particles not diffolved, '"pwfca folu- 

 but merely fufpended in confequence of their extreme divi- ' 

 ft on ; it is alfo certain, that under other circumftances the 

 defect of tranfparency depends intirely upon the incomplete 

 folubility of one or more bodies contained in thefe fluids, fo 

 that in order to give them the defired limpidity, we muft ne- 

 ceffarily have recourfe to the methods which increafe the fo- 

 lubility of the fubftanees in queftion, or effect its total fepa- 

 falion. 



Albumen 



