Letter | Published:

Osmotic Pressure

Nature volume 54, pages 571572 | Download Citation



IN the October number of the Philosophical Magazine will be found an interesting paper, by Prof. Poynting, which explains the phenomena of the osmotic pressure of solutions by the hypothesis of chemical combination between the solvent and the dissolved matter. I wish to direct the attention of your readers to one point in the paper, and to a development of it which seems to me to be worthy of notice. Any successful theory of solution must explain the fact that the osmotic pressure obeys the usual laws of gaseous pressure—those of Boyle and Avogadro—and, moreover, has the same absolute value as that of the pressure which the dissolved molecules would exert in the gaseous state, when filling a volume equal to that of the solution. It has always been clear that, whatever be the ultimate cause of the osmotic pressure, the gaseous laws must be obeyed by dilute solutions. The molecules of any finely-divided matter must be, in general, out of each other's sphere of influence, so that each will produce its effect independently of the rest. But this is all that is necessary for Boyle's law and Avogadro's law to hold, so that these, as well as the mere existence of osmotic pressure, are explained by chemical combination just as well as by molecular bombardment. On the other hand, no good reason has been hitherto given why chemical forces should be so adjusted that the osmotic pressure of the dissolved molecules should have the same absolute value as that of the pressure which the same number of gaseous molecules would exert when filling an equal volume.

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  1. Trinity College, Cambridge, October 12.

    • W. C. D. WHETHAM


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