To obtain these from plants or peels, the articles should be infused for two or three days, or even longer, in a sufficient quantity of cold water, until it has fully penetrated the pores of the materials. For this purpose roots should be cut into thin slices, barks reduced to a coarse powder, and seeds slightly bruised; those of soft and loose texture require to be infused two or three days, the harder and more compact a week or two, whilst, some tender herbs and plants require to be distilled directly. After the solvent has fully penetrated, distil it with an open fire; that is, a fire under the still like a common washing copper, which immediately strikes the bottom. Regulate the fire so as to make it boil as speedily as possible, and that the oil may continue to distil freely during the whole process; for the longer it is submitted to an unnecessary heat without boiling, a greater portion of the oil is mixed with the water than there would otherwise be. The oil comes over the water, and either sinks to the bottom or swims on the top, according as it is lighter or heavier than that fluid. What comes over at first is more fragrant than that towards the end, which is thicker, and should be re-distilled by a gentle heat, when it leaves a resinous matter behind.

All essential oils, after they are distilled, should be suffered to stand some days in open bottles or vessels, loosely covered with paper to keep out the dust, until they have lost their disagreeable fiery odour, and become quite limpid: put them into small bottles, and keep them quite full in a cold place. The light oils pass over the swan neck of the common still, but the heavier ones will not so readily, therefore a large low head is preferable; the heavier oils are those from cloves, allspice, cinnamon, etc, or such as contain a portion of resin.

Some plants yield three times as much oil, if gathered when the flowers begin to fall off, - as lavender; others when young, before they have sent forth any flowers, - as sage; and others when the flowers begin to appear, - as thyme.

All fragrant herbs yield a large portion of oil when produced in dry soils and warm summers. Herbs and flowers give out a larger quantity of oil after they have been partly dried in a dry shady place. Four pounds of the leaves of the dried mint yield one ounce of oil, but six pounds of fresh leaves only three drachms and a-half. This oil is more fine and bright when rectified - that is, re-distilled.

After the distillation of one oil, the worm should be carefully cleansed, by passing a little spirit of wine through it, before another is proceeded with.

A great quantity of oil is wasted by confectioners when they preserve their lemon and orange peels by boiling them in open vessels instead of a still; what is saved by this means alone would soon repay the expense of the apparatus.