PHARMACY. 



by agaiu condensing this vapour in a pro- 

 per apparatus. Water i& converted into 

 vapour at 212, and may be separated by 

 distillation from the earthy and saline mat- 

 ters which it always contains in a natural 

 state. But, it is evident, that if any sub- 

 stances which are as Volatile as water, be 

 exposed to the same degree of heat, 

 either by immersing them in boiling wa- 

 ter, or exposing them to the action of its 

 steam, they will rise with it in distillation. 

 In this way the camphor and volatile oils 

 of vegetable substances are separated 

 from the more fixed principles ; and as 

 water is capable of dissolving a certain 

 quantity of these volatile substances, it 

 may be impregnated with a great variety 

 of Havours by distilling it from different 

 aromatic substances. If the subject of 

 our distillation contain more volatile oil 

 than the water employed is capable of 

 dissolving, it will render the water milky, 

 and afterwards separate from it. Ii is in 

 this way that essential oils are obtain- 

 ed. 



Essential oils are obtained only from 

 odoriferous substances ; but not equally 

 from all of this class, nor in quantity pro- 

 portional to their degree of odour. Some, 

 which, if we were to reason from analo- 

 gy, shou'd seem very well fitted for this 

 process, yield extremely little oil, and 

 others none at all Roses and camo- 

 mile flowers, whose strong and lasting 

 smell promises abundance, are found to 

 contain but a small quantity of oil ; the 

 violet and jessamine flower, which per- 

 fume the air with their odour, lose their 

 smell upon the gentlest coction, and do 

 not afford any oil, on being distilled, un- 

 less immense quantities are. submitted to 

 the operation at once ; while savin, whose 

 disagreeable scent extends to no great 

 distance, gives out the largest propor- 

 tion of oil of almost any vegetable known. 



Nor are the same plants equally fit for 

 this operation, when produced in differ- 

 ent soils or seasons, or at different times 

 of their growth. Some yield more oil, if 

 gathered when the flowers begin to fall 

 off, than at any other time. Of this we 

 have examples in lavender and rue ; 

 others, as sage, afford the largest quan- 

 tity when young, before they have sent 

 forth any flowers ; and others, as thyme, 

 when the flowers have just appeared. All 

 fs-agrant herbs yield a larger proportion 

 of oil, when produced in dry soils and in 

 warm summers, than in opposite circum- 

 stances. On the other hand, some of the 

 disagreeable strong-scented ones, as 

 worm-wood, are said to contain most oil 



in rainy seasons, and when growing in 

 moist rich grounds. 



Several chemists have been of opinion, 

 that herbs and flowers moderately dried, 

 yield a greater quamity of essential oil, 

 than if they were drilled when fresh. It 

 is, however, highly improbable, that the 

 quantity of essential oil will be increased, 

 by drying ; on the contrary, part of it must 

 he dissipated and lost. But drying may 

 sometimes be useful in other ways ; eithec 

 by diminishing the bulk of the subject to 

 be distilled, or by causing it to part with 

 its oil more easily. 



The choice of proper instruments is of 

 great consequence for the performance of 

 this process to advantage. There are 

 some oils which pass freely over the 

 swan-neck of the head of the common 

 still : others, less volatile, cannot easily be 

 made to rise so high. For obtaining 

 these last, we would recommend a large 

 low head, having a rim or hollow canal 

 round it : in this canal the oil is detain- 

 ed in its first ascent, and thence con- 

 veyed at once into the receiver, the ad- 

 vantages of which are sufficiently obvi- 

 ous. 



With regard to the proportion of water 

 to be employed ; if whole plants, mode- 

 rately dried, are used, or the shavings of 

 woods, as much of either may be put in- 

 to the vessel, as, lightly pressed, will oc- 

 cupy half its cavity ; and as much water 

 may be added as will fill two thirds of it. 

 When fresh and juicy herbs are to be dis- 

 tilled, thrice their weight of water will 

 be fully sufficient ; but dry ones require a 

 much larger quantity. In general, there 

 should be so much water, that after all 

 intended to be distilled has come over, 

 there may be liquor enough left to prevent 

 the matter from burning to the still. The 

 water and ingredients, altogether, should 

 never take up more than three fourths of 

 the still ; there should be liquor enough 

 to prevent any danger of an empyreuma, 

 but not so much as to be apt to boil over 

 into the receiver. 



The subject of distillation should be 

 macerated in the water until it be per- 

 fectly penetrated by it. To promote this 

 effect, woods should be thinly shave 

 across the grain, or sawn, roots cut trai 

 versely into thin slices, barks reduced ii 

 to coarse powder, and seeds slight* 

 bruised. Very compact and tenackn 

 substances require the maceration to 

 continued a week or two, or longer ; 

 those of a softer and looser texture, 

 or three days are sufficient ; while 

 tender herbs and flowers not only 



