CHAPTER IV. 



THE MEMBRANES. 



Pfeffer was the first to give to artificial semi-permeable membranes 

 the support of a rigid background, and to him, therefore, belongs the 

 credit of having originated the only practicable method of measuring 

 directly and correctly the osmotic pressure of solutions. But many 

 years of persistent investigation were necessary in order to overcome 

 the great difficulties which are inherent in the method and to reduce it 

 to a workable form through the elimination of its sources of error. 



The following is an accurate restatement, though not an entirely 

 literal translation, of Pfeffer's description of his method of depositing 

 membranes : 



"The clay cells were first completely injected with water by means of 

 repeated evacuations under the air pump. They were then filled with, and 

 placed for several hours in, a 3 per cent solution of copper sulphate. After- 

 wards, they were several times quickly rinsed (upon the inside only) with 

 water, and well dried internally and as expeditiously as possible with strips 

 of filter paper. Having been dried slightly upon the outside, they were left 

 exposed in the air until the exterior surface was still just moist to the feel. 

 The cells were then filled with a 3 per cent solution of potassium ferrocyanide, 

 and returned to the solution of copper sulphate. 



" After standing quietly from 24 to 48 hours, the cells were filled completely 

 with a solution of ferrocyanide and closed. The contents now developed grad- 

 ually a certain over-pressure due to the superior osmotic pressure of the interior 

 solution. After another period of from 24 to 48 hours, the apparatus was 

 opened and refilled with a solution containing 3 per cent of potassium ferro- 

 cyanide and 1.5 per cent of saltpeter (by weight), which developed ordinarily 

 an osmotic pressure of something more than 3 atmospheres. If the cells were 

 to be used for higher pressures, they were tested with solutions containing 

 more saltpeter." 



Pfeffer found that a "slow increase in pressure and a certain period 

 of low pressure" were essential to success in the preparation of his mem- 

 branes. The explanation which he gives is that, at certain points, the 

 membrane spans depressions in the cell wall, and is, therefore, more 

 liable to rupture, if the pressure, which is to force the membrane against 

 the wall in such unsupported places, is rapidly or suddenly increased. 

 He also states that a somewhat prolonged period of deposition is neces- 

 sary in order to give to the membrane the strength which will enable it 

 to withstand pressure. 



The attempts which were made in this laboratory many years ago to 

 reproduce the membranes of Pfeffer with a view to measuring osmotic 

 pressure, especially that of concentrated solutions, were failures, as 

 have been all similar attempts on the part of other investigators. It 

 was not impossible to make membranes which would yield impressive 



77 



