= Solution
\b[<Decoction> suits material whose useful constituents are water-soluble and withstand boiling; <infusion> avoids sustained boiling and can better preserve delicate or volatile constituents.] The choice depends on both the plant's structure and the chemistry of the constituent being extracted.
Hard <bark>, <wood> and many kinds of <root (botany)> need comminution and more prolonged contact with hot <water> to penetrate their tissues; a <decoction> can assist extraction when the desired material is stable. Boiling is unsuitable when it drives off a needed <volatile oil>, decomposes the active constituent, or extracts an unwanted excess of <starch> or other material. A hard <root (botany)> is therefore not automatically an instruction to boil. Period examples suitable for <decoction> include <oak bark>, <logwood> and <pareira root>; appropriate <infusions> include aromatic <cloves> and <chamomile>. Cold <infusion> of <calumba> avoids dissolving its <starch>, illustrating that suitability depends on the desired selectivity of <solvent extraction>, not merely on whether a drug is a leaf or a root. <The Essentials of Materia Medica and Therapeutics by Alfred Baring Garrod (1865)>[Relation of measures to weights, part 2, Infusa and cold calumba; corresponding British plant monographs]{ref}
For an <infusion>, the appropriately divided drug is left in a measured solvent for a prescribed time, commonly in a covered vessel. Hot <water> assists extraction, while a cover reduces loss of volatile matter. Cold <infusion> can be chosen when heat is undesirable. The result must be strained or filtered; time, temperature, particle size and solvent quantity affect its strength. Neither method guarantees that every constituent is extracted in the same proportion.
\b[A <liquid extract> is made by extracting the specified drug with a suitable solvent, separating the liquid from the exhausted material, and bringing the extract to its prescribed concentration and volume.] <Maceration> can precede <percolation>; expression or washing recovers retained liquid. A percolator passes fresh solvent through a packed bed, rather than indefinitely soaking the drug in an increasingly saturated liquid. Where concentration is necessary, controlled evaporation limits damage; an alcoholic fraction can be reserved or recombined as the monograph directs. Different <liquid extracts> can use aqueous, alcoholic or ethereal processes. For example, the <liquid extract> of <male fern> is an ethereal extraction of a resinous and oily fraction, whereas the <liquid extract> of <opium> is compounded from a previously made aqueous extract. These are chemically different operations. There is no warrant for assigning all 1864 extracts one modern universal drug-to-extract ratio. <The Prescriber's Analysis of the British Pharmacopoeia by John Birkbeck Nevins (1864)>[pp. 20–21 and 42–43, differing liquid extracts and strengths; p. 15, percolation]{ref}
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