We use no manual labour after preparing the apparatus, and have no pressed residue to pulverise. We simply connect maceration as before examined to one of nature's most familiar laws, and in this latter experiment have an exemplification of the process which Prof. Procter recommended for the preparation of fluid extracts and tinctures. It is only a modification of the processes previously examined, differing in the manner in which the liquid is separated from the powder. It is simple in operation, easy in manipulation, and productive of satisfactory results when properly applied. It is called percolation, under which name we shall perhaps be led to examine some points of interest connected therewith, and some modifications which have been suggested as improvements over Prof. Procter's process, very properly denominated simple percolation.
Prof. Procter, in bringing before pharmacists this process to deplete a powder of soluble matters, laid no claim to originality, excepting in the application of the principle for the purpose of making tinctures and fluid extracts. He certainly was aware that the process had been in use for a similar process was recommended by Count Rumford for preparing coffee; and in 1817, C. Johnson applied the principle to the extraction of cinchona bark, saying: "The machine I use is similar to one made several years ago by Edmund Loyd and Co., 178, Strand, and does not differ essentially from any of those described in Count Rumford's eighteenth essay, and in the Repertory of Arts, for April and May, 1813." Of the practical application of the process, Johnson remarks," that in the Lancaster Public Dispensary this method is found to afford a better preparation than whs formerly obtained from twice the quantity of cinchona."
Pelouze, as early as 1834, introduced percolation into the laboratory of the chemist in his method of preparing tannic acid, calling it "extraction by the process of displacement." Virtually, percolation had been employed for ages before with civilised and even partly barbarous nations, as, for example, in making saltpetre and potash.
In conducting percolation, the object being the preparation of fluid extracts, many points are essential other than the considerations mentioned heretofore. Of those the most essential to be considered are the vessel employed, the material operated upon, the menstruum used, and the manner of manipulation. Accepting the argument that percolation is for the economical extraction of soluble materials, it is of the utmost importance to study influence of contact between the menstruum and the material whose partial solution is to be effected, as we have already seen that contact, continued for a length of time, is of first necessity. Thus, if we place 1 lb. of powdered sugar, or any other soluble substance, within a cylindrical percolator of such diameter that the space occupied is 1 in. in height, and cautiously add, evenly upon the upper surface, diluted alcohol, admitting for the sake of argument that the menstruum passes evenly and regularly through the powder, the diluted alcohol in the first of the percolate will have been in contact with 1 in. of material. That which follows will have successively less material to operate upon, for the first portions of percolate are partly made up of dissolved sugar or extracted matter.
Thus each preceding portion of the percolate lessens the material in the percolator, and lessens the height, thus decreasing the contact of any that may succeed, until finally only a thin layer of sugar remains, between which and the passing menstruum the contact is very slight indeed. At last the sugar disappears. For this reason, even where the material is completely soluble, our percolate should theoretically become less and less charged with dissolved matters as percolation progresses (unless it be saturated to a certain point of the percolate), and at last a comparatively large amount of menstruum should contain but a small amount of dissolved material.
Let us now imagine a like amount of powdered sugar in a percolate of less diameter. The height will be increased and the contact between the first part of percolate and powder will be greater in proportion to the increased height. Allowing, for argument, the material to occupy 8 in. in height, it will follow that the menstruum of the first portion of percolate will have passed through 8 times the height of sugar that the corresponding portion did in the former experiment, although the real amount of sugar was the same. Now, again, we have the afore-mentioned rule regarding decrease of contact. Each successive part of the percolate lessens the sugar in the percolator, and decreases the possible contact (with sugar) of all the menstruum that may follow, and under like motion of liquid the sugar decreases in each succeeding part of the percolate. It will be seen that, theoretically, each portion of the menstruum in the smaller percolator must have greater contact with the material than the corresponding menstruum of the larger, if both percolate with the same rapidity, although in both examples we operate upon similar amounts of material.
Arguing therefrom we are induced to anticipate that unless the percolate from the percolator of greater diameter is saturated with sugar, that which corresponds from the smaller will contain more dissolved matter, for after 1 in. - the depth of sugar in the percolator of greatest diameter - is passed there remains in the smaller 7 in. of contact during which solution may progress. Calculating accordingly we may expect that if we spread 1 lb. of sugar so that it will occupy a depth of 1 in. in a percolator, and percolate through it diluted alcohol enough to produce 16 fl. oz. of percolate, we will fail to obtain as much sugar in solution as though the sugar had been placed in a vessel of less diameter, thereby increasing the contact between menstruum and sugar. Applying the same rule to larger and smaller amounts of other substances, we must conclude that unless there be counterbalancing influences the amount of dissolved matter in a percolate must increase and decrease with alteration in the height of powder, other conditions being identical, and amount of percolate passing from each in a given time.