"In conducting a series of experiments on percolation, Dr. Squibb (1866) proved, first, that there is a sufficient degree of uniformity of results to admit of the adoption of a model plan of proceeding, applicable to drugs in general; second, that the extract or soluble matter yielded to the menstruum is not uniform in its chemical and therapeutical value, as obtained during the different stages of the percolation, but diminishes in effective value far more rapidly than the extract does in weight; and third, that this decrease in value depends upon the difference in solubility between the active and inactive portions of the extract, and that the ratio of this decrease is about the same for drugs in general, provided the proper menstruum be used. Critical experiments made with seven different drugs proved that the first 12 fluid ounces of the percolate contained from 61 to 78 per cent, of the total extract obtainable from 16 troy ounces of a drug with 3 or 4 pints of percolate, as directed by the Pharmacopoeia of 1860, and that the first 16 fluid ounces contained from 71 to 84 per cent, of the total amount of extract. Subsequently (1869) Samuel Campbell showed that some drugs may be practically exhausted by careful management on obtaining 1 fluid ounce for every troy ounce of the drug used. This principle, however, is applicable to those drugs only which are rather heavy, and at the same time are readily permeated by a menstruum in which the active principles are easily and freely soluble, so that complete exhaustion is attained without difficulty. The United States Pharmacopoeia, 1870, directed in most cases 24 fluid ounces of percolate for 16 troy ounces of drug, and regarded the latter then as practically exhausted, The present Pharmacopoeia directs displacement to be continued until the drug is exhausted, a point which may be reached by careful manipulation without requiring finally long-continued evaporation. But while for fluid extracts the exhaustion of the material is mostly left to the good judgment of the operator, definite quantities of percolate are usually directed for the extracts, sufficient to exhaust the material completely. In the light of the experience cited above, it would seem that this might likewise have been left to the judgment of the experienced operator. When the active principles possess a decided taste, their gradual diminution in the percolate is easily ascertained; in other cases recourse may be had to chemical tests, especially in the presence of alkaloids. The color of the percolate alone is no reliable criterion for its medicinal strength, some drugs continuing to yield colored percolates after the active principles have been exhausted, while others still yield appreciable quantities of the latter after most of the coloring matter has been taken up. The strength of extracts and fluid extracts depends solely on the amount of the active principles, and not on the total amount of extractive matter obtained.

"For moistening the powder, a definite quantity of menstruum is now directed which experience has shown to be most suitable; generally this quantity is greater than was formerly thought desirable. The manner of packing described is the most convenient, the pressing of the whole amount of the powder in one operation giving better and more uniform results in the hands of' most operators than if done in fractions. Ligneous material requires to be very firmly packed, and when of more loose cellular structure the packing should be less firm, but in all cases it should be uniform. Again, when an alcoholic menstruum is used, the packing must be correspondingly firmer as the menstruum is stronger in alcohol. When well packed, a disc of paper or muslin is spread upon the surface, and the requisite menstruum poured upon it. Thus arranged, the material is permitted to macerate for 2 days, both orifices of the percolator being securely closed to prevent evaporation. The time directed for maceration is ample, but may be prolonged to 3 or 4 days without disadvantage, or for easily-exhausted drugs even shortened. It is to be observed that the powder is to remain constantly covered by a stratum of the menstruum; this precaution is of imperative necessity, until the powder has been deprived of the greater proportion of its active principles, and the liquid in the percolator may be more than sufficient to displace the solution of the remainder.