Page:Popular Science Monthly Volume 16.djvu/522

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496
THE POPULAR SCIENCE MONTHLY.

the operations performed by the early chemists: some state in a general way their acquaintance with the processes of distillation, sublimation, filtration, etc.; others are more explicit. Hermann Kopp, in his exhaustive "Geschichte der Chemie,"[1] states that "filtration as a chemical process was first accurately described by Geber, who calls it by a special name, destillatio per filtrum, 'trickling through a filter,' in contrast to the collection of a liquid by ordinary distillation." Ferdinand Hoefer, in his "Histoire de Chimie"[2] writing of Geber, mentions that he recognizes two kinds of distillation—with and without fire—the former being "per alembicum," and the latter consisting of "une simple filtration."

Now, we propose to show that the ancients carried on the operation of filtration in two ways, essentially distinct in principle and in the manner of execution, and that these methods were characterized by two different expressions which have been confounded by the authors named. Moreover, we shall establish this by quotations from writings covering a period of more than two thousand years.

In the first place, an examination of the very passage in Geber's Works, referred to by Kopp and by Hoefer, shows that the method therein described differs radically from filtration as ordinarily conducted at the present time. We quote the passage as found in the works of Geber, "the most famous Arabian Prince and Philosopher," "faithfully englished by R[ichard] R[ussell]," and printed at London in 1678. In the thirteenth chapter of the fourth part of the first book of the "Summe of Perfection," Geber treats of the three kinds of distillation: by an "Alembeck," by a "Decensory," and "by Filter." After describing in quaint language the well-known method of using the alembic and the decensory (which differs chiefly in the application of heat on the top of the apparatus), Geber writes thus of filtration: "The Disposition of that which is made by Filter is, that the Liquor to be Distilled be put into a Stone Concha, and the wider part of the Filter put into the said Liquor, even to the Bottom of the Concha, but the narrower part of it hang out over the Orifice of the said Vessel. And under that end of the Filter must be set another Vessel for receiving the Distillation. Therefore when the Filter begins to Distill, the Water with which it was moistened will first Distill off; which ceasing, the Liquor to be Distilled succeeds. Which Liquor, if it be not as yet serene, it must so often be put into the Concha again and redistilled, as until it be Distilled most serene."

This dates from the eighth century, and evidently describes a sort of capillary siphoning. The expressions placing the wider part of the filter into the liquid and allowing the "narrower part of it to hang out over" the vessel admit of no other interpretation. For convenience of distinguishing this method of filtration from that in which porous sacs are employed, we propose to name the former

  1. Vol. ii., p. 26.
  2. Vol. i., p. 335.