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The microscopic structure of semipermeable membranes and the part played by surface forces in osmosis
Aug., I916.] FUTURE OF CHEMISTRY IN }tlGI[ SCtf~)OL.
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from that uncertain criterion, newspaper reports--to be forced back to the use of soda ash. The sane thing seems to be to add interest by the aid of technology, to keep the applications of chemistry to human needs before the students, but to avoid over-emphasis. Special attention should be paid to those cases in which a technical process exemplifies and clears up an important general principle. An illustration is the connection of catalysis with the manufacture of sulphuric acid, both in the lead chambers and by the contact process. (d) In all these recommendations there is nothing revolutionary and nothing radically new. In Germany the abandonment of the dogmatic analytical procedure began with Arendt (1862) and Wilbrand (187o), and has been complete for a generation. In England the same movement has progressed under the leadership of Perkin and Armstrong. Since the basic idea is to proceed logically from the concrete to the abstract--and, where possible, to return to the concrete at the end of the train of thought to clinch the principle by an application significant to the student--the plan leads to an unusual attention to household matters, familiar phenomena, every-day affairs. Thus far are we in accord with those who would have us make our teaching more practical. But we wish to interpret the word not with the myopic vision of the half-educated man, who despises everything which he cannot understand, but with the wide and imaginative outlook of the man who knows that the disinterested research of to-day is the factory commonplace of to-morrow. The commercial history of the world for the last fifty years points the moral that the nation which fails to grasp the value of science, and especially of chemical science, is moving to disaster. America should learn this lesson now, and not through an industrial catastrophe hereafter. The Microscopic Structure of Semipermeable Membranes and the Part Played by Surface Forces in Osmosis. F. TINKER. (Proceedings of the Royal Society, Series A, vol. 92, No. A64I, May 6, IgI6.)--Hitherto very few experimental observations have been made to arrive at some of the fundamental facts associated with the mechanism of osmosis. Raoult, Flusin, Kahlenberg, and a few others have shown, however, that certain membranes, such as parchment, gelatine, and rubber, absorb the liquids to which they are permeable, and are impermeable to liquids which they do not absorb;
248
CURRFN[" TOPICS.
[J. F. I.
Bigelow and Bartell have shown that, under certain conditions, the rate of flow of water through a membrane such as copper ferrocyanide obeys Poiseuille's law for the rate of flow through capillary tubes; while Beutner, Donnan, and others have proved that certain precipitatim.l membranes can act as electrodes, reversible with respect to various ions. The further questions which are dealt with in the present communication are experimental ones, such as: W h a t is the size of the colloidal particles of which a semipermeable membrane is composed, and how is the membrane built up from those particles ? T o what extent does a membrane show the properties of the gelatinous precipitates or gels as ordinarily prepared by bulk precipitation ? H o w is the structure of the membrane altered by variations in the method of its formation, the nature of the solutions bathing it, and the treatment to which it is subjected? and what is the size of its pores, the extent to which they are under the control of the surface forces and therefore of absorption phenomena also ? ( I ) The common precipitation semipermeable membranes are composed of small precipitate particles, ranging from o.I/, to I.O/~, these particles being packed closely together. Each of these precipitate particles is, however, not simple in structure, but is itself an aggregate formed by the flocculation of semimicroscopic colloidal particles. The particles composing the membrane are smallest in the case of copper ferrocyanide and Prussian blue. (2) Precipitation membranes show most of the properties of gels, as ordinarily prepared, both in their method of formation and in the changes they undergo in various solutions. Like ordinary gels, they are possessed of great tensile strength, which varies in membranes of various kinds. Their stability in the colloidal condition also varies greatly. But, although they show the physical properties of gels, they have not the same mechanical structure, the membrane being much more Closely knit together than the gel proper. (3) The pores in a copper ferrocyanide membrane range from 8 to 6 o ~ in diameter, the average diameter being from 15 to 2of*t*. The pore size is too great for the membrane to act osmotically by exerting a selective blocking action. (4) The order of a series of membranes in pore size is the same as that of their efficiency" as semipermeable membranes. Copper ferrocyanide and Prussian blue are the most efficient membranes, and they have also the smallest pores. (5) There is also very close connection between the osmotic properties of a membrane and the extent to which the membrane capillaries are under the control of surface forces. Osmotic effects are probably the result of selective absorption phenomena occurring at the surface of the membrane and in the capillaries, the membrane being relatively impermeable to solutes which are negatively absorbed, but permeable to solutes which are positively absorbed.
247
from that uncertain criterion, newspaper reports--to be forced back to the use of soda ash. The sane thing seems to be to add interest by the aid of technology, to keep the applications of chemistry to human needs before the students, but to avoid over-emphasis. Special attention should be paid to those cases in which a technical process exemplifies and clears up an important general principle. An illustration is the connection of catalysis with the manufacture of sulphuric acid, both in the lead chambers and by the contact process. (d) In all these recommendations there is nothing revolutionary and nothing radically new. In Germany the abandonment of the dogmatic analytical procedure began with Arendt (1862) and Wilbrand (187o), and has been complete for a generation. In England the same movement has progressed under the leadership of Perkin and Armstrong. Since the basic idea is to proceed logically from the concrete to the abstract--and, where possible, to return to the concrete at the end of the train of thought to clinch the principle by an application significant to the student--the plan leads to an unusual attention to household matters, familiar phenomena, every-day affairs. Thus far are we in accord with those who would have us make our teaching more practical. But we wish to interpret the word not with the myopic vision of the half-educated man, who despises everything which he cannot understand, but with the wide and imaginative outlook of the man who knows that the disinterested research of to-day is the factory commonplace of to-morrow. The commercial history of the world for the last fifty years points the moral that the nation which fails to grasp the value of science, and especially of chemical science, is moving to disaster. America should learn this lesson now, and not through an industrial catastrophe hereafter. The Microscopic Structure of Semipermeable Membranes and the Part Played by Surface Forces in Osmosis. F. TINKER. (Proceedings of the Royal Society, Series A, vol. 92, No. A64I, May 6, IgI6.)--Hitherto very few experimental observations have been made to arrive at some of the fundamental facts associated with the mechanism of osmosis. Raoult, Flusin, Kahlenberg, and a few others have shown, however, that certain membranes, such as parchment, gelatine, and rubber, absorb the liquids to which they are permeable, and are impermeable to liquids which they do not absorb;
248
CURRFN[" TOPICS.
[J. F. I.
Bigelow and Bartell have shown that, under certain conditions, the rate of flow of water through a membrane such as copper ferrocyanide obeys Poiseuille's law for the rate of flow through capillary tubes; while Beutner, Donnan, and others have proved that certain precipitatim.l membranes can act as electrodes, reversible with respect to various ions. The further questions which are dealt with in the present communication are experimental ones, such as: W h a t is the size of the colloidal particles of which a semipermeable membrane is composed, and how is the membrane built up from those particles ? T o what extent does a membrane show the properties of the gelatinous precipitates or gels as ordinarily prepared by bulk precipitation ? H o w is the structure of the membrane altered by variations in the method of its formation, the nature of the solutions bathing it, and the treatment to which it is subjected? and what is the size of its pores, the extent to which they are under the control of the surface forces and therefore of absorption phenomena also ? ( I ) The common precipitation semipermeable membranes are composed of small precipitate particles, ranging from o.I/, to I.O/~, these particles being packed closely together. Each of these precipitate particles is, however, not simple in structure, but is itself an aggregate formed by the flocculation of semimicroscopic colloidal particles. The particles composing the membrane are smallest in the case of copper ferrocyanide and Prussian blue. (2) Precipitation membranes show most of the properties of gels, as ordinarily prepared, both in their method of formation and in the changes they undergo in various solutions. Like ordinary gels, they are possessed of great tensile strength, which varies in membranes of various kinds. Their stability in the colloidal condition also varies greatly. But, although they show the physical properties of gels, they have not the same mechanical structure, the membrane being much more Closely knit together than the gel proper. (3) The pores in a copper ferrocyanide membrane range from 8 to 6 o ~ in diameter, the average diameter being from 15 to 2of*t*. The pore size is too great for the membrane to act osmotically by exerting a selective blocking action. (4) The order of a series of membranes in pore size is the same as that of their efficiency" as semipermeable membranes. Copper ferrocyanide and Prussian blue are the most efficient membranes, and they have also the smallest pores. (5) There is also very close connection between the osmotic properties of a membrane and the extent to which the membrane capillaries are under the control of surface forces. Osmotic effects are probably the result of selective absorption phenomena occurring at the surface of the membrane and in the capillaries, the membrane being relatively impermeable to solutes which are negatively absorbed, but permeable to solutes which are positively absorbed.