Traube artificial cells and precipitation membranes

Leduc reported that Traube of Breslau discovered in 1867 that osmotic membranes could be made artificially, and that precipitates such as copper ferrocyanide form membranes with properties like those of osmotic membranes². Darwin, in Insectivorous Plants, cited Traube's experiments on artificial cells and their permeability to salts, and indexed Traube's work on artificial cells¹. Verworn described the gelatine and tannin version. A drop of thick gelatine solution, dropped carefully into tannin, acquires a precipitation membrane of gelatine tannate³.

Verworn used the gelatine drop to illustrate growth of superficial structures in living cells. He wrote that it was much discussed in Traube's time, and that its likeness to a living cell earned it the name artificial cell³. Water enters the drop and swells it. Fine holes and cracks that appear are closed by new precipitate. By his account the artificial cell therefore enlarges steadily and uniformly until all the gelatine is combined⁴.

Authors disagreed on the structure. Leduc held that Traube's picture, a solid mass with small orifices that closes larger ones by precipitate and grows by intussusception, was refuted. He cited his own work and that of Reinke, Cohn and de Vries, who saw the membranes regress and turn friable⁵. Leduc also called the term semi-permeable inexact, since no membrane passable to water fully blocks solutes⁶. Bayliss discussed Walden's ranking of many membranes. Tannin-gelatine was the most permeable and copper ferrocyanide the least⁷. Bayliss also cited Bartell, who found water flow through copper ferrocyanide followed Poiseuille's formula for capillary tubes⁸.

Lillie used precipitation membranes as a guide to the plasma membrane. He judged that living semi-permeable membranes are probably comparable to well-supported copper ferrocyanide membranes of the kind Morse employed⁹. He added that artificial colloidal membranes deteriorate in electrolytes, while living ones are apparently kept intact by factors tied to metabolism, and that breaks are presumably repaired at once by new structural material⁹.