tions, 



532. ON THE fROCRSSES VOR CLARIFYING LIQUIDS, 



Various precau- But while we confider the nature of filters, their form and 

 fituation are no lefs important. In order that a filter of paper 

 may produce its full effe&, it mutt not adhere by its whole 

 ftrface againit that furface which fupports it, for if this were 

 the cafe the nitration would be foon Hopped. This inconve- 

 nience is avoided by dividing it in different directions. But 

 as the folds are foon flattened, fome prefer placing between 

 the fupport and the filter, flravvs or fimple tubes of glafs. I 

 muft confefs that this laft method has not always fucceeded 

 with me, and that I have moil commonly obferved, that the 

 folds made in niters produce nearly as much effect as the 

 pieces of flraw and tubes. In Germany (and in England) 

 they have funnels grooved or fluted within for thus ufe. 



Whatever may be the precautions which have been taken, 

 a period will arrive at which the nitration becomes flow, and 

 at length totally Hops. This effect takes place when the pores 

 of the paper are fo much obflrucled, that they no longer ad- 

 mit the paffage of the fluid. Sometimes the filtration may be 

 prolonged by giving a flight circular motion to the funnel ; but 

 this effect is of fhort duration, and there is no other remedy 

 but to change the filter ltklf. It feems that there has not yet 

 been found any means of remedying this inconvenience, which 

 is common to all niters whatever. 

 Filtres of wool- We have before obferved that niters are made of woollen 

 ' cloth, of piece goods, and of carded cotton. The woollen 



were formerly much ufed, and were even thefirft niters adop- 

 ted. They were formed into the figure of a cone, of which 

 the bafe was kept open by an hoop that was fixed in a frame 

 with fupporters. This kind of filter is ftill ufed to filter rata- 

 fias, as it may be rendered very capacious, and is fufceptible 

 of receiving a large quantity of fluid at once; but it affords 

 little, and it is therefore neceflary to wait a long time before 

 the fluid paffes clear ; for which reafon it is feldom ufed, ex- 

 / cepting when no other apparatus can be procured, 



and their ufes. Neverthelefs, when the filtration of fyrups is required, 

 woollen cloths are ufed ; but then inftead of giving them the 

 form of a cone, the operator fimply fixes his cloth in a fquare 

 frame, fattening the four corners upon pins difpofed for that 

 purpofe. The boiling fyrup is poured in the middle, where 

 it always forms a kind of concavity, and frequently after a 

 few minutes the liquid panes very clear. 



3 This 



