Page:Text-book of Electrochemistry.djvu/54

 lower osmotic pressure. Sea- weed has, as a rule, an osmotic pressure greater by about 4 atmos. than the water sur- rounding it.

Tammann's Measurements. — The method devised by Tammann (8) is capable of more general application. The method consists in observing the diffusion currents in Topler's apparatus. If in a liquid the density is greater or less at any one point than in the neighbourhood, then at this point the liquid either falls or rises, and this is easily seen on account of the different refractive indices of the solutions of different densities. Thereby the well-known phenomenon of streaking is produced, a phenomenon which is often to be noticed when the sun shines on a wall, producing a slow upward current of air.

Tammann uses as bath a solution of potassium ferro- cyanide. Into this is introduced a drop of a solution of a copper or zinc salt hanging from the end of a glass point ; the drop at once becomes covered with a semi-permeable membrane of copper or zinc ferrocyanide. If the drop is isotonic with the solution in the bath, no change of concen- tration takes place in its neighboiu*hood, and no streaking is formed. If the drop is more concentrated than the ferro- cyanide solution, it absorbs water from the solution in immediate contact with it, thus making part of the solution specifically heavier than the mass of liquid in the bath, and it sinks, producing a streakiness along its course. The opposite action takes place when the solution within the membrane is too dilute.

In Tammann's experiments, the drops contained in some cases other substances, such as ethyl alcohol, cane sugar, etc., besides the copper salt. The osmotic pressure of the sub- stances was calculated on the assumption that the total osmotic pressure is the sum of the osmotic pressure of the copper salt and that of the substance added.

The following table, which contains the chief results obtained by Tammann, gives the isotonic coefficients of those solutions which correspond with 01 n and 0*3 n solutions of

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