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HARRY C. JONES 



rise in temperature of 1°C. the volume being kept constant. 

 This raises the question, does osmotic pressure increase with 

 rise in temperature and if so, how much? The measurements of 

 Pfeffer enable us to answer this question. He not only measured 

 the osmotic pressure of solutions of cane sugar at different con- 

 centrations, but the osmotic pressure of a given solution of cane 

 sugar at different temperatures. 



The temperature coefficient of osmotic pressure as calculated 

 from PfefTer's work is approximately frs', but this is only an 

 approximation; there being comparatively wide deviations in 

 the results obtained by Pfeffer bearing on this question. 



Fortunately there is an indirect method of testing whether the 

 law of Gay-Lussac applies to the osmotic pressure of solutions. 

 This method is based upon a principle discovered by Soret. MTien 

 all of the different parts of a homogeneous solution are main- 

 tained at the same temperature, the whole solution will remain 

 of homogeneous concentration ; that is, all the different parts 

 will have the same concentration. 



If, however, one part of the solution is heated to a different 

 temperature from the remainder, the warmer part becomes more 

 dilute and the colder part more concentrated. This is known as 

 the principle of Soret. 



If Gay-Lussac 's law applies to the osmotic pressure of solutions, 

 then since, as we shall see, osmotic pressure is the cause of all 

 diffusion, we should be able to calculate the difTerence in con- 

 centration of different parts of a solution heated to different tem- 

 peratures, from the difference in temperature of the different 

 parts of the solution in question. 



A moment's reflection will show this to be true. The warmer 

 solution becomes more dilute because the osmotic pressure of a 

 warm particle is greq,ter than that of a cold particle. This excess 

 of osmotic pressure drives the dissolved substance over from the 

 region of greater pressure to the region of lesser pressure, from the 

 warmer to the colder part of the solution — and this continues until 

 there is an equality of osmotic pressure in all parts of the solution. 

 If the difference in temperature of the two parts of the solution is, 

 say 50°, then the colder solution should be more concentrated 



