PHARMACY. 



educed either by the combustion of other 

 substances, or by the partial combustion 

 of the substance to be charred. In the 

 former case, the operation may be per- 

 formed in any vessel which excludes the 

 access of air, while it permits the escape 

 of the vapours formed. In the latter, the 

 access of air must be regulated in such a 

 manner, that it may be suppressed when- 

 ever the combustion has reached the re- 

 quisite degree ; for if continued to be ad- 

 mitted, the charcoal itself would be dissi- 

 pated in the form of carbonic acid gas, 

 and nothing would remain but the alka- 

 line and earthy matter, which these sub- 

 stances always contain. When combus- 

 tion is carried this length, the process is 

 termed incineration. The vapours which 

 arise in the operation of charring, are 

 sometimes condensed, as in the manufac- 

 ture of tar. 



Evaporation is the conversion of a fluid 

 into vapour, by its combination with ca- 

 loric. In this process, the atmosphere is 

 not a necessary agent, but rather a hin- 

 drance, by its pressure. This forms a cri- 

 terion between chemical evaporation and 

 spontaneous evaporation, which is mere- 

 ly the solution of a fluid in air. It is per- 

 formed in open, shallow, or hemispheri- 

 cal vessels of silver, tinned copper or 

 iron, earthenware or glass. The neces- 

 sary caloric may be furnished by means 

 of an open fire, a lamp, or a furnace, 

 either immediately, or with the interven- 

 tion of sand, water, or vapour. The de- 

 gree of heat must be regulated by the na- 

 ture of the substance operated on. In ge- 

 neral, it should not be greater than what 

 is absolutely necessary. 



Evaporation may be partial; producing, 

 from saline fluids, concentration; and 

 from viscid fluids, inspissation; or it may 

 be total, and produce exsiccation. Con- 

 centration is employed to lessen the 

 quantity of diluting fluids, which is called 

 dephlegmation ; or as a preliminary step 

 to crystallization. Inspissation is almost 

 confined to animal and vegetable substan- 

 ces ; and as these are apt to be partially 

 decomposed by heat, or to become em- 

 pyreumatic, it should always be perform- 

 ed, especially towards the end of the pro- 

 cess, in a water or vapour bath. Exsicca- 

 tion is here taken in a very limited sense ; 

 for the term is also with propriety used 

 to express the drying of vegetables by a 

 gentle heat, the efflorescence of salts, 

 and the abstraction of moisture from mix- 

 tures of insoluble powders with water, 

 by means of chalk-stones, or powdered 

 chalk pressed into a smooth mass. At 



present, we limit its meaning to the total 

 expulsion of moisture from any body by 

 means of caloric. The exsiccation of 

 compound oxides should always be per- 

 formed in the water bath. Salts are de- 

 prived of their water of crystallization by 

 exposing them to the action of heat in a 

 glass vessel or iron ladle. Sometimes 

 they first dissolve in their water of crys- 

 tallization, or undergo what is called the 

 watery fusion, and are afterwards con- 

 verted into a dry mass by its total expul- 

 sion ; as in the calcination of borax or 

 burning of alum. When exsiccation is at- 

 tended with a crackling noise, and split- 

 ting of the salt, as in muriate of soda, it 

 is termed decrepitation, and is performed 

 by throwing into a heated iron vessel 

 small quantities of the salt at a time, co- 

 vering it up, and waiting until the decre- 

 pitation be over, before a fresh quantity 

 is thrown in. Exsiccation is performed 

 on saline bodies, to render them more 

 acrid or pulverulent, or to prepare them 

 for chemical operations. Animal and ve- 

 getable substances are exsiccated, to give 

 them a solid form, and to prevent their 

 fermentation. 



Condensation is the reverse of expan- 

 sion, and is produced either by mechani- 

 cal pressure forcing out the caloric in a 

 sensible form, as water is squeezed out 

 of a sponge, or by the chemical abstrac- 

 tion of caloric, which is followed by an 

 approximation of the particles of the sub- 

 stance. This latter kind alone is the ob- 

 ject of our investigation at present. In 

 this way we may be supposed to condense 

 substances existing naturally as gases or 

 vapours ; or substances, naturally solid 

 or fluid,, converted into vapours by adven- 

 titious circumstances. The former in- 

 stance is almost supposititious ; for we 

 are not able, by any diminution of tempe- 

 rature, to reduce the permanently elastic 

 fluids to a fluid or solid state. The lat- 

 ter instance is always preceded by vapori- 

 zation, and comprehends those operations 

 in which the substances vaporized are 

 condensed in proper vessels. 



When the product is a fluid, it is term- 

 ed distillation ; when solid, sublimation. 

 Distillation is said to be performed, vid 

 humidd, when fluids are the subjects of 

 the operation. Via sicca, when solids are 

 subjected to the operation, and the fluid 

 product arises from decomposition, and a 

 new arrangement of the constituent prin- 

 ciples. The objects of distillation are, to 

 separate more volatile fluids from less 

 volatile fluids or solids: to promote the 



