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XIII. On the Osmotic Pressures of Calcium Ferrocyanide Solutions. 



Part II. Weak Solutions. 



By the EARL OF BERKELEY, F.R.S., E. G. J. HARTLEY, B.A. (Oxon.), 

 and J. STEPHENSON, B.Sc. (Land.). 



Received January 16, Read February 18, 1909. 



IN the following communication the experiments on the direct measurements of the 

 osmotic pressure and the densities have been carried out in conjunction with 

 Mr. HARTLEY, while those on the electric conductivities were with Mr. STEPHENSON. 



The reason for choosing calcium ferrocyanide as solute were given in Part I., but it 

 may be added that weak solutions of this salt showed certain interesting anomalies, 

 to be described later, which made it desirable to measure their electric conductivities 

 with a view to throwing some light on the connection between osmotic pressure and 

 ionisation. 



The process followed in purifying the salt has already been given in Part I.,"" and 

 eo has that used in obtaining the densities. 



The Measurements of Osmotic Equilibrium Pressures. 



These measurements are confined to solutions whose equilibrium pressures are under 

 25 atmospheres. The apparatus was the same as that previously described in the 

 account of our experiments on sugar solutions (' Phil. Trans.,' Series A, vol. 206, p. 482), 

 except that the large dead-weight pressure-gauge was replaced by a smaller one 

 specially made by Messrs. Schiiffer and Budenberg, which seemingly allowed accurate 

 measurements to be obtained down to 2 '5 atmospheres pressure, and at the same time 

 the increment of pressure (loc. cit., p. 498) was O'l atmosphere. t 



It had been thought that by increasing the area of the membrane (but keeping the 

 genei'al construction of the apparatus the same) these lower pressures would be 

 measurable to a greater degree of accuracy that is to say, that for a small change 

 in pressure a larger quantity of water would pass through the membrane and 

 consequently give a bigger effect on the reading of the water-gauge. Some 50 

 porcelain tubes, supposed to be similar to those used previoiisly in paste porosity 

 and diameter (but 20 cm. in length), were therefore obtained from the makers. 

 Unfortunately all efforts to obtain even one membrane good enough for accurate work 

 failed, and the attempt has had to be abandoned for the time being ; consequently the 

 experiments were done with the short tubes. 



* ' Phil. Trans.,' Series A, vol. 209, p. 179. 



t This apparatus was compared with the mercury column at the National Physical Laboratory and 

 found correct. 



VOL. CCIX. A 453. 26.5.09 



