550 SCIENCE PROGRESS 



pressures up to 150 atmospheres. 1 The porous pot z was 

 46 mm. high and 16 mm. wide with walls \\ to 2 mm. thick. 

 The glass tubes v and t were joined to the porous pot by 

 two layers of shellac, the upper hard, the lower a little soft, 

 in order to make a sound joint. The softening of the shellac at 

 higher temperatures (up to 2>7°) was compensated by the 

 addition of a glass ring r filled with cement which held the 

 apparatus rigidly together and by a layer of the same cement 

 above the shellac in the joints between v and t; the cement used 

 was the well-known mixture of litharge and glycerol. Before 

 depositing the membrane the porous pot was extracted with 

 potash and with chlorhydric acid and freed from air by soaking 

 in water and evacuating with an air-pump. The pot was soaked 

 during several hours in a 3 per cent, solution of copper sulphate, 

 rinsed internally with water and partially dried with filter- 

 papers and by exposure to the air, then filled with a 3 per cent, 

 solution of potassium ferrocyanide and immersed again in the 

 copper-sulphate solution. After standing during twenty -four to 

 forty-eight hours the cell was closed and exposed to the pressure 

 due to the osmosis of the membrane-forming solutions ; twenty- 

 four to forty-eight hours later it was emptied, charged with 

 a 3 per cent, ferrocyanide containing ih per cent, of saltpetre 

 and exposed to the osmotic pressure of 3 atmospheres which 

 this solution develops or if necessary to the higher pressure 

 developed by a stronger solution. Membranes of Prussian blue 

 were prepared in the same way by using ih per cent, ferric 

 chloride outside and 3 per cent, ferrocyanide inside the cell, 

 whilst membranes of calcium phosphate were prepared from a 

 3 per cent, solution of calcium chloride and a 6 per cent, 

 solution of disodium phosphate neutralised with sodium bicar- 

 bonate. Membranes of ferric hydroxide and ferric phosphate 

 were also tried. 



It will be seen from the description that has been given not 

 only that Pfeffer's experiments were very extensive in their 

 range but that they were carried out with very considerable 

 care. It is, indeed, noteworthy that his methods were adopted 

 almost in toto by Morse twenty-five years later and that nearly 

 all the sources of error which the American workers strove so 

 long and so successfully to eliminate had been recognised (and 



1 Pfeffer records pressures up to 436"8 cm., i.e. nearly 6 atmospheres, in the 

 case of a y^ P er cent, solution of saltpetre in water, 



