PHARMACY. 



minute division of bodies. It is perform- 

 ed in fiat mortars, of glass, agate, or other 

 hard materials, by giving a rotatory mo- 

 tion to ihe pestle; or on a levigating 

 stone, which is generally of porphyry, by 

 means of a muller of the same substance. 

 On large quantities it is performed by 

 rollers of hard stone, turning horizontally 

 upon each other, or by one vertical roller 

 turning on a fiat stone. 



The substances subjected to this ope- 

 ration are generally previously powdered 

 or ground. 



Levigatiwt differs from triluration only 

 in the addition of water or spirit of wine 

 to .the powder operated upon, so as to 

 form the whole mass into a kind of paste, 

 which is rubbed until it be of sufficient 

 smoothness or fineness. Earths and some 

 metallic substances are levigated. 



Granulation is employed lor the me- 

 chanical division of some metals. It is 

 performed, either by stirring the melted 

 metal with an iron rod until it cools, or 

 by pouring it into water, and stirring it 

 continually as before, or by pouring it 

 into a covered box, previously well rubbed 

 with chalk, and shaking it until the metal 

 cools, when the rolling motion will be 

 converted into a rattling one. The ad- 

 hering chalk is then to be washed away. 



Jtfsckanical Separation is obtained by 

 sifting, elutriation, decantation, filtration, 

 despumation, expression. 



Sifting. From dry substances, which 

 are reduced to the due degree of minute- 

 ness, the coarser particles are to be sepa- 

 rated by sieves of iron-ware, hair cloth, 

 or gauze, or by being dusted through 

 bags of fine linen. For very light and va- 

 luable powders, or acrid substances, com- 

 pound sieves, having a close lid and 

 receiver, must be used. The particles 

 which are not of sufficient fineness to 

 pass through the interstices of the sieve, 

 may be again powdered. 



Elutriation is confined to mineral sub- 

 stances, on which water has no action. It 

 is performed by separating them from 

 foreign particles and impurities, of a dif- 

 ferent specific gravity, in which case they 

 are said to be washed ; or for separating 

 the impalpable powders, obtained by tri- 

 turation and levigation from the coarser 

 particles. This process depends upon 

 the property that very fine or light pow- 

 ders have of remaining for some time 

 suspended in water ; and is performed by 

 diffusing the powder or paste formed by 

 levigation through plenty of water, let- 

 ting it stand a sufficient time, until the 



coarser particles settle at the bottom ; 

 and then pouring off the liquid in which 

 the finer or lighter particles are suspend- 

 ed. Fresh water may be poured on the 

 residuum, and the operation repealed ; or 

 the coarser particles, which fall to the 

 bottom, may be previously levigated a se- 

 cond time. 



Decantation. The fine powder which is 

 washed over with the water is separated 

 from it, by allowing it to subside com- 

 pletely, and by either decanting off the 

 water very carefully, or by drawing u off 

 by a syringe or syphon These processes 

 are very frequently made use of for sepa- 

 rating fluids from solids which are -speci- 

 fically heavier, especially when the quan- 

 tity is very large, or the solid so subtle as 

 to pass through the pores of most sub- 

 stances employed for filtration, or the 

 liquid so acrid as to corrode them 



Filtration. For the same purpose of se- 

 parating fluids from solids, straining and 

 filtration are often used. These differ 

 only in degree, and are employed when 

 the powder either dot- s not subside at all, 

 or too slowly and imperfectly for decanta- 

 tion. The instruments for this purpose 

 are of various materials, and must in no 

 instance be acted upon by the substances 

 for which they are employed. Fa's, re- 

 sins, wax, and oils, are strained through 

 hemp or flax, spread evenly over a piece 

 of wire-cloth or net stretched in a frame. 

 For saccharine and mucilaginous liquors, 

 fine flannel may be used : for some saline 

 solutions, linen. Where these are not fine 

 enough, unsized paper is employed ; but 

 it is extremely apt to burst by hot watery 

 liquors, which dissolve its size ; and very 

 acrid liquors, such as acids, are filtered by 

 means of a glass-funnel, filled with pow- 

 dered quartz, a few of the larger pieces be- 

 ing put into the neck, smaller pieces over 

 these, and the finer powder placed over 

 all. The porosity of this last filter retains 

 much of the liquor ; but it may be reco- 

 vered by gently pouring on it as much 

 distilled water ; the liquor will then pass 

 through, and the water be retained in its 

 place. 



Water may be filtered in large quanti- 

 ties through basins of porous stone, or 

 artificial basins of nearly equal parts of 

 fine clay and coarse sand. The siie of 

 the filters depends on the quantity of 

 matter to be strained. When large, the 

 flannel or linen is formed into a conical 

 bag, and suspended from a hoop or frame; 

 the paper is either spread on the inside of 

 these bags, or folded into a conical form, 



