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XV. Some Experiments on the Diffusion of Substances in 

 Solution. By Spencer Umfreville Pickering, M.A., 



F.E.S.* 



HPHE following determinations of the comparative rates of 

 _L diffusion of various non-electrolytes were intended to be 

 preliminary to a more extended investigation, but, as I do not 

 see my way at present to continue the work, I think it well 

 to put on record those results which have so far been obtained. 



Osmotic pressure is held to be due simply to the gaseous 

 impact of the dissolved substance, and is therefore proportional 

 to the number of molcules per unit volume, the temperature 

 being constant. Now, by Graham's law, the rate of diffusion 

 of a gas should be, cceteris paribus, inversely as the square root 

 of its density, or of its molecular weight. Hence, at a given 

 temperature and pressure the product of the molecular 

 weight, m, by the square of the rate of diffusion, v, should be 

 a constant. The relative values for v in the case of different 

 bodies might be determined by simple diffusion-experiments 

 if these could be conducted under perfect conditions: namely, 

 the diffusing liquid remaining of a constant strength through- 

 out, and the water into which diffusion is taking place 

 remaining uncontaminated with the substance. The present 

 results seem to show, either that the above supposition as to 

 osmotic pressure is incorrect, or that the conditions obtainable 

 in diffusion-experiments are very far from obtaining to ideal 

 perfection ; for the values deduced for mv 2 are by no means 

 constant, even in those cases where the substances behave 

 normally as to their osmotic pressure, i.e. where the solutions of 

 equal molecular strength give the same osmotic pressure, this 

 latter being measured by the depression exercised by them on 

 the freezing-point of the water in which they are dissolved. 



The method adopted was that termed by Graham jar- 

 diffusion. The solutions under examination were placed in 

 an open jar inside a large vessel of water, and the amount of 

 substance which had diffused out of the former in a given 

 time was ascertained by determining the strength of the 

 remaining solution. The inner jars were beakers of a cylin- 

 drical form with ground flanges : they were ground also on 

 the bottom, and rested on inverted ground-glass saucers 

 placed in the centre of the large outer glass jars. Before 

 being rilled, each jar was placed in position and adjusted so 



* Communicated lw the Author. 



