Let us not infer, however, that the conditions cannot render the foregoing to an extent inaccurate. If our material be placed loosely in the percolator as a consequence the first portion of menstruum will pass rapidly. If after the first fraction of percolate is obtained the flow be retarded by means of a stopcock, that which follows may be held in contact with the material some time longer than the first; after the second fraction is reserved the flow may be again retarded, and thus more actual contact of time induced between menstruum and material than was obtained at first, although there is continually less material within the percolator. With some substances another benefit to be derived by the latter percolates arises from the fact that if the material be not finely divided or pressed firmly into the percolator the first portion of percolate flows over the particles and through the interstices between, thus preventing the menstruum flow coming into close contact with soluble materials. Gradually, however, the material may absorb menstruum, and expanding fill up those interstices, thus forcing the passing percolate to seek more and more the capillary passages through the material, and thus give a larger amount of dissolved material to a portion of percolate succeeding a certain amount of the first.
To an extent this result may occur from a somewhat similar cause, even with materials perfectly soluble in the menstruum, as, for an example, sugar or salt. With small amounts of loosely packed granulated sugar the first part of a percolate of diluted alcohol or water quickly finds the exit of the percolator, but the surfaces of the particles are in the meantime softened and the mass contracts. The interstices become filled with thick syrup or solution, and thus the percolates that follow are for a time retarded. It will be noticed that the foregoing discrepancies result simply from imperfect contact, or, as we may say, imperfect maceration.
We will now consider another phase of the subject. Will a certain amount of material, occupying a height of 10 in., yield to corresponding portions of percolate less dissolved matter than a smaller amount in a percolator of such size as to make the height 20 in.? If we accept the foregoing arguments we must conclude this will be the case to a certain point of the operation, unless the percolate from each percolator, is saturated, as each drop of menstruum passing through the one will come into contact with a larger portion of material than that from the other, until a certain amount of soluble matter is carried from the smallest amount of material, when it will naturally follow that the percolate from the largest amount of material will contain more dissolved matter. In other words, the first portion of percolate from the material occupying the greatest height will excel the other, while afterwards the case will be reversed. Perpendicular height should govern to this extent the result from this standpoint regardless of quantity.
In considering now that phase of contact between menstruum and solid, called maceration, in connection with percolation, one cannot find any influence at work arising from a force other than those simply due to a prolongation of contact before considered. The passing menstruum is retarded, thus permitting a longer time for the action of the solvent. In treating of this entire subject let as bear constantly in mind that our aim is to dissolve solid substances, and that the various modifications of the processes are simply influences affecting solution.
If we close the exit of our percolator at any time during the progress of percolation, the menstruum within the percolator will necessarily cease to move bodily downward. The liquid will thus remain in direct contact with the material, and as a consequence the act of solution will progress in a manner similar to that exemplified by our example of the dissolving crystal of potassium bromide. Hence, it is evident that no other advantage than those resulting from longer continued contact can arise. To guard against any disturbing influence affecting succeeding percolation, caused by an unequal contraction of the only partially saturated powder, it is to be observed that all particles of material are equally and permanently surrounded by menstruum. We must bear in mind that the action of the menstruum upon the powdered material in the percolator, which consists of a number of small fragments, and that upon the single crystal of potassium bromide, in the example cited heretofore, differ only in degree; its solvent power affects alike all the molecules exposed to Its influence, and the relative difference is dependent solely upon the difference of the areas of surface exposed to contact.
In fact, the term molecule implies no definite idea of size, and is an expression applying to something beyond our senses; we cannot compare the molecules of a liquid to particles of matter of any conceivable size. We are forced to assume that a menstruum is made up of an inconceivably large number of infinitely small particles, which we consider capable of permeating the powder within the percolator, finding its way through the capillary channels which surround the particles of the solid, circulating around them in obedience to laws already considered, and according to influences yet to be mentioned.
During the process of maceration in the percolator the capillary tubes, as well as the larger interstices, are supposed to be filled with liquid; if this liquid be capable of dissolving wholly or partially the solid, solution must take place. Each successive movement of contact is found to decrease the quantity of matter held in solution until the liquid is saturated or the solid dissolved. Thus we find the effect of contact in percolation to be identical with that in simple maceration.