=~ BOTANIC PHYSICIAN. 
Besides this species of crystallization, there is a process of a simi- 
_ lar kind to which the same term is applied. When a body has 
been melted, if the fluid has its temperature slowly reduced, so 
that it approach gradually to the point at which it becomes solid, its 
part a, unite, so as to form masses of regular figures, or 
rst ‘This species of crystallization is not however; applied to 
y pharmaceutical purpose. binds 
Precipitation is another process by which a solid body is sepa- 
rated from a fluid. If, for example, to the solution of any solid mat+ 
ter in any fluid, a substance be added, which has a stronger attrac- 
tion to the fluid than the solid previously dissolved in it has, the lat- 
ter will be separated, and will be thrown down in its solid form. In like 
“manner, precipitation may take place from the substance which is 
added combining not with the fluid, but with the solid dissolved, and 
forming with it a new compound no longer soluble. The process, 
her case, is termed precipitation, the substance thrown down 
cipitate, and the substance by which the precipitation has 
, is the precipitant. 
clear liquor is poured off, the precipitate is repeatedly washed, to 
carry off any of the substance by which it was precipitated, which 
‘The preceding operations are those in which a solid body dissolved 
in a fluid is recovered by abstracting part of the fluid. But there are 
Many cases of combination, or decomposition, in which our object 
is rather to obtain the more volatile part of the mixture. In such — 
cases distillation and sublimation are had recourse to. : 
In DISTILLATION, the materials are exposed to a proper degree of 
heat, in vi rived so as to collect the part which passes off 
in vapor, condense it, and thus obtain it in the fluid form. Distillae ~ 
tion is therefore nothing but- evaporation in close vessels, with the 
_ design of obtaining the volatile products. i 
in many cases of distillation, the matter to be distilled is aptto 
is the case with regard to the essential oilsof plants. To prevent 
this, they are not exposed alone to heat, but’ with the addition of @ 
quantity of water. The temperature at which water is conver 
into vapor, (212° of Fahren heit) is sufficient ‘to volatilize these oils, 
without injuring them. The oil therefore, rises. with the vapor, is 
‘condensed with it, and is easily separated from the water, by their 
difference in specific gravity, a small quantity of it only being lost by 
being retained by the water in solution. oe 2 
_~ When the substance to be distilled is not of such a nature as to” 
act chemically on metallic vessels, the common still, made of ironor 
copper, isemployed. This isa cylindrical vessel, from the head of 
which a pipe issues, which is connected with a spiral tube, place ‘in 
a vessel filled with water. The heat is directly applied to the bot-_ 
tom of the still; the vapor produced passes off through the tube, and © 
is condensed in the spiral tube (or worm as it is termed,) the cold 
water with which this is surrounded promoting the condensation. 
Acids, or other substances that act upon metallic vessels, or 
