PHARMACY. 



fervescence. The gas is very frequently 

 allowed to escape into the atmosphere, 

 but at oiher times is either collected in a 

 pneumatic apparatus, or made to enter 

 into some new combination. The vessels 

 in which an effervescing mixture is made, 

 should be high and sufficiently large, to 

 prevent any loss of the materials from 

 their running over, and in some cases the 

 mixture must be made slowly and gradu- 

 ally. 



Precipitation is the reverse of solution. 

 It comprehends all those processes in 

 which a solid is obtained by the decom- 

 position of a solution. The substance se- 

 parated is termed a precipitate, if it sink 

 to the bottom of the fluid ; or a cream, if 

 it swim above it. Precipitation, like so- 

 lution, is performed either via humidd, or 

 via siccd ; and is effected by lessening the 

 quantity of the solvent by evaporation ; 

 by diminishing its powers, as by reduc- 

 tion of temperature, or dilution ; or by 

 the addition of some chemical agent, 

 which, from its more powerful affinities, 

 eitaer combines with the solvent, and 

 precipitates the solve nd, or forms itself 

 an insoluble compound with some con- 

 stituent of the solution. 



The objects of precipitation are, the 

 separation of substances from solutions in 

 which they are contained ; the purifica- 

 tion of solutions from precipitable impu- 

 rities ; or the formation of new combina- 

 tions. 



The two first means of precipitation 

 have been already noticed. In perform- 

 ing it in the last manner we may observe 

 the following rules : The solution and 

 precipitant must possess the requisite de- 

 gree of purity. The solution should be 

 perfectly saturated, to avoid unnecessary 

 expenditure of the solvent or precipitant. 

 The one is to be added slowly and gradu- 

 ally to the other. After each addition, 

 they are to be thoroughly mixed by agi- 

 tation We must allow the mixture to 

 settle after we think that enough of the 

 precipitant has been added, and try a lit- 

 tle of the clear solution, by adding to it 

 some of the precipitant ; if any precipita- 

 tion takes place, we have not added 

 enough of the precipitant. This is ne- 

 cessary, not only to avoid loss, but in ma- 

 ny instances the precipitant, if added in 

 excess, re-dissolves or combines with the 

 precipitate. 



After the precipitation is completed, 

 the precipitate is to be separated from 

 the supernatant fluid by some of the 

 means already noticed. 



When the precipitate is the chief ob- 



ject of our process, and when it is not 

 soluble in water, it is often advisable to 

 dilute to a considerable degree both the 

 solution and precipitant before perform- 

 ing the operation. When it is only diffi- 

 cultly soluble, we must content ourselves 

 with washing the precipitate after it is 

 separated by filtration. In some cases 

 the separation of the precipitate is much 

 assisted by a gentle heat. 



Crystallization is a species of precipita- 

 tion, in which the particles of the solvend, 

 on separating from the solution, assume 

 certain determinate foi'ms. The condi- 

 tions necessary for crystallization are, that 

 the integrant particles have a tendency 

 to arrange themselves in a determinate 

 manner, when acted on by the attraction 

 of aggregation; that they be disaggre- 

 gated, at least so far as to possess suffi- 

 cient mobility to assume their peculiar 

 arrangement; and that the causes dis- 

 aggregating them be slowly and gradually 

 removed. 



Notwithstanding the immense variety 

 in the forms of crystals, M. Hauy has 

 rendered it probable that there are only 

 three forms of the integrant particles ; the 

 parallelepiped, the triangular prism, and 

 the tetrahedron. But as these particles 

 may unite in different ways, either by 

 their faces or edges, they will compose 

 crystals of various forms. 



The primitive forms have been reduced 

 to six; the parallelepiped, the regular 

 tetrahedron, the octahedron with triangu- 

 lar faces, the six-sided prism, the dode- 

 cahedron terminated by rhombs, the do- 

 decahedron with isosceles triangular faces. 



Almost all substances on crystallizing 

 retain a portion of water combined with 

 them, which is essential to their existence 

 as crystals, and is therefore denominated 

 water of crystallization. Its quantity varies 

 very much in different crystallized sub- 

 stances. 



The means by which the particles of 

 bodies are disaggregated, so as to admit 

 of crystallization, are solution, fusion, 

 vaporization, or mechanical division and 

 suspension in a fluid medium. The means 

 by which the disaggregating causes are 

 removed are evaporation, reduction of 

 temperature, and rest. 



When bodies are merely suspended in 

 a state of extreme mechanical division, 

 nothing but rest is necessary for their 

 crystallization. When they are disaggre- 

 gated by fusion or vaporization, the regu- 

 larity of their crystals depends on the 

 slosviiess with which their temperature 

 is reduced; for if cooled too quickly, their 



