1904.] Method of Measuring High Osmotic Pressures. 439 



The porcelain tube, AA, was made of Dr. Puckal's paste by the 

 .Royal Berlin Factory, and of a porosity slightly less than that of the 

 porous pots they make for Dr. Thorpe's arsenic apparatus. It is 

 about 15 cm. long, and has an outside diameter of 2*5 cm., and an 

 inside diameter of 1 cm. The ends are glazed and the membrane was 

 formed on the whole length of the outer surface. The outside brass 

 casing BB, contained the solution, to which the pressure was brought 

 by the steel pressure tube C. The porcelain tube is secured to B by 

 the " dermatine " rings DD, and the latter are strongly compressed, 

 between B and the metal sleeves EE, by screwing down the nuts FF, 

 the lateral expansion causes them to grip the tube tightly. GG- are 

 holes perforated through B, and through the sleeves EE, the use of 

 which will be explained later on. H is another perforation through B 

 which allows the apparatus to be emptied without taking it to pieces ; 

 in several experiments a Schaffer and Budenberg pressure-gauge was 

 attached here, ordinarily it is closed by a screw-down metal plug. 



The ends of the porcelain tube are closed by pieces of thick-walled 

 rubber tubing II, through which the brass tubes JJ pass ; the length 

 of rubber is such that they extend further into the tube than the 

 distance between the " dermatine " rings and the ends of the tube. 

 A watertight joint between A and J is obtained by compressing the 

 rubber tubes between the metal washers KK, and the screw sleeves 

 LL. The brass tubes J J are joined, in one case to a glass tap, and in 

 the other to a glass capillary by rubber tubing. The glass capillary is 

 graduated in millimetres and was calibrated; 1 cm. of the bore 

 contains -0042 cc. 



The operation of measuring the osmotic pressure of a solution 

 consisted in filling the inside of the porcelain tube with water, and the 

 surrounding vessel B with the solution ; then noting the rate at which 

 the level of the water in the graduated tube moves while the pressure 

 is being gradually increased on the solution. 



Theoretically, as long as the osmotic pressure has not been reached, 

 the level of the water in the capillary should fall ; when the osmotic 

 pressure is exceeded it should rise and the "turning point" should 

 give the osmotic pressure. In either case the rate at which it moves 

 is a function of the difference between the osmotic pressure and the 

 pressure on the solution. In the actual experiments, although the 

 level rose and fell, the " turning point " was at some other pressure 

 higher than the osmotic pressure ; the chief cause of this difference 

 was that however tightly the " dermatine " rings were compressed 

 against the porcelain, the sugar solution leaked past them. This was 

 doubtless because the membrane could not be formed quite on the 

 outer surface of the tube, and consequently a very narrow ring of the 

 tube was left open. The leak was so small that no difficulty was 

 experienced in keeping the pressure up, even at 50 atmospheres, but 



