PHARMACY. 



union of different substances : and to 

 generate new products by the action of 

 fire. 



In all distillations, the heat applied 

 should not be greater than what is ne- 

 cessary for the formation of the va- 

 pour, and even to this degree it should 

 be gradually raised. The vessels also 

 in which the distillation is performed 

 should never be filled above one-half, and 

 sometimes not above one-fourth, lest the 

 substance contained in them should boil 

 over. 



As distillation is a combination of eva- 

 poration and condensation, the apparatus 

 consists of two principal parts : the ves- 

 sels in which the vapours are formed ; 

 and those in which they are condensed. 

 The vessels employed for both purposes 

 are very various in their shapes, accord- 

 ing to the manner in which the operation 

 is conducted. The first difference de- 

 pends on the direction of the vapour after 

 its formation. It either descends, as- 

 cends, or passes off by the side, constitut- 

 ing 1 a distillation /? descenswn, per ascen- 

 sum, or per laius. 



In the distillation per descenswn, a per- 

 forated plate of tinned iron, or other ma- 

 terials, is fixed within any convenient ves- 

 sel, so as to leave a space beneath it. On 

 this the subject of the operation is laid, and 

 over it is placed another plate, accurately 

 closing the mouth of the vessel, and suf- 

 ficiently strong to support the fuel : thus 

 the heat is applied from above, and the 

 vapour is forced to descend into the infe- 

 rior cavity, where it is condensed. In this 

 way the oil of cloves is prepared, and on 

 the same principles tar is manufactured, 

 and mercury and zinc are separated from 

 their ores. 



In the distillation per ascenswn, the va- 

 pour is allowed to rise to some height, and 

 then is conveyed away to be condensed. 

 The vessel most commonly employed for 

 this purpose is the common copper still, 

 which consists of a body for containing 

 the materials, and a head into which the 

 vapour ascends. From the middle of the 

 head a tube rises for a short way, and is 

 then reflected downwards, through which 

 the steam passes to be condensed. Ano- 

 ther kind of head, rising to a great height 

 before it is reflected, is sometimes used 

 for separating fluids, which differ little in 

 volatility, as it was supposed that the less 

 volatile vapours would be condensed and 

 fall back into the still, while only the 

 more volatile vapours- would arise to the 

 top, so as to pass to the refrigeratory. 

 The same object may be more conve- 



niently attained by managing the fire with 

 caution and address. The greater the sur- 

 face exposed, and the less the height the 

 vapours have to ascend, the more rapidly 

 does the distillation proceed ; and so 

 well are these principles understood by 

 the Scotch distillers, that they do not take 

 more than three minutes to discharge u. 

 still containing gallons of fluid. 



The condensing apparatus used with 

 the common still is very simple. The 

 tube in which the head terminates is in- 

 serted into the upper end of a pipe, which 

 is kept cool by passing through a vessel 

 filled with water, called the refrigeratory. 

 This pipe is commonly made of a serpen- 

 tine form ; but as this renders it difficult 

 to be cleaned, Dr. Black recommends a 

 sigmoid pipe. The refrigeratory may be 

 furnrehed with a stop-cock, that when the 

 water it contains becomes too hot, and 

 does not condense all the vapour produc- 

 ed, it may be changed for cold water. 

 From the lower end of the pipe, the pro- 

 duct of the distillation drops into the ves- 

 sel destined to receive it ; and we may 

 observe, that ^vhen any vapour issues 

 along with it, we should either diminish 

 the power of the fire, or change the wa- 

 ter in the refrigeratory. There was a 

 process of this kind, called circulation. It 

 consisted in arranging the apparatus, so 

 that the vapours were no sooner condens- 

 ed into a fluid form, than this fluid- re- 

 turned buck into the distilling vessels, to 

 be again vaporised ; and was effected by 

 distilling in a glass vessel, with so long a 

 neck that the vapours were condensed be- 

 fore they escaped at the upper extremity, 

 or by inverting one matrass within ano* 

 ther. When corrosive substances are 

 distilled in this way, the cucurbit and 

 alembic are used; but these substances 

 are more conveniently distilled per la- 

 tus. 



The distillation per latus is performed 

 in a retort, or pear-shaped vessel, having 

 the neck bent to one side. The body of a 

 good retort is well rounded, uniform in 

 its appearance, and of an equal thickness, 

 and the neck is sufficiently bent to allow 

 the vapours, when condensed, to run 

 freely away, but not so much as to ren- 

 der the application of the receiver incon- 

 venient, or to bring it too near the fur- 

 nace. The passage from the body into 

 the neck must be* perfectly free and suf- 

 ficiently wide, otherwise the vapours pro- 

 duced in the retort only circulate in its 

 body without passing over into the re- 

 ceiver. For introducing liquors into the 

 retort without soiling its neck, which 



