616 Extracts. part m.

EXTRACTA.

Extracts.

These are preparations obtained by evaporating aqueousand alcoholic solutions of vegetable substances, until a mass ofa somewhat firm tenacious consistence remains. When waterhas been employed for making the solution, the extract mayconsist of gum or mucilage, extractive, tannin, cinchonin, sac-charine matter, and the salts which the vegetable contained, andis termed a Watery Extract; but if alcohol has been the men-struum, resin, extractive, and all the above matters, except thegum, may be the ingredients, and the extract is denominateda Spirituous Extract. The latter appellation also is used if proofspirit be employed. The proper menstruum, therefore, for thepreparation of any extract must be that fluid which most rea-dily dissolves the peculiar principles on which the medicinal ef-ficacy of the vegetable is supposed to depend.

When water is to be employed, the substance to be subjectedto its action should be in the dried state, and coarsely pow-dered ; and the solution, whether made by decoction or infusion,should be evaporated immediately after it is strained, and whilstit is yet hot; for, as we observed in treating of Decoctions,water at the temperature of 212° takes up much more of theactive matter of vegetables than it can hold in solution at alower temperature: therefore, by allowing them to cool, withthe view of defecation, and evaporating the clear fluid only,a considerable portion of the active matter does not enter intothe extract, and is necessarily lost. In performing the evapo-ration, a higher temperature than that of boiling water mustnot be employed; but it must, nevertheless, be conducted asquickly as possible; and therefore the evaporating vessel shouldbe broad and shallow, and set in boiling water, or the water-bath recommended by Dr. Powell 1 should be employed.(See Instruments, Part i.)

Alcohol is used only in cases where the active ingredient ofthe vegetable is chiefly resin, or too volatile to bear the heatwhich is necessary for evaporating the water without being dis-sipated, or without suffering some decomposition, which wouldmaterially alter its properties. A tincture of the substance isfirst obtained, which is then evaporated by a very gentle heat

1 Translation nfihe London Pharmacopoeia, p 201.