OF VITAL PHENOMENA 33 



the right temperature by immersing it in ice and salt; it is then 

 wiped dry and pushed through the bored cork of a larger test 

 tube and returned to the freezing mixture. The thermometer 

 should not touch the wall of the inner test tube, and this in turn 

 should not touch the outer test tube, as the local cooling thus 

 produced might cause a local freezing before the entire solution 

 reached the freezing point. The solution is stirred until freezing 

 commences. The temperature will fall below the freezing point 

 and when the first ice separates suddenly rise to the freezing 

 point. This undercooling may be lessened by having the freez- 

 ing mixture only slightly below the freezing point of the solution 

 or by dropping in a crystal of ice to start the freezing at the 

 proper moment. Since the sudden separation of ice that occurs 

 before the definitive freezing point is reached concentrates the 

 remaining solution, the freezing point lowering is too great and 

 should be decreased 1/80 for every degree of undercooling. 



Although the cryoscopic method is very simple in principle, 

 it is only by the most painstaking performance that an accuracy 

 of .001° is obtained. Since a A of 1.85 corresponds to an 

 osmotic pressure of 22.4 atmospheres, .001 ° represents a pressure 

 of .0912 atmospheres or 9.2 mm of mercury, and is very serious 

 when small differences in osmotic pressure are to be measured. 

 It should be remembered that the cryoscopic method determines 

 the osmotic pressure at the freezing point of the solution, and 

 this should be reduced to the desired temperature by correcting 

 for the effect of the temperature change on the electrolytic disso- 

 ciation and osmotic pressure. 



A method for determining the freezing point of small amounts 

 of solution, devised by Barger (1904), has been called a micro- 

 tensimeter, but it is in reality an osmometer in which air forms 

 the semipermeable membrane. It consists of a capillary tube 

 that can be observed under a microscope having an ocular mi- 

 crometer. The osmotic pressure of the solution is approximately 

 known from previous data or a preliminary test. A graded series 

 of solutions is prepared, the extremes of which will fall on the 

 two sides of the osmotic pressure of the unknown. Drops of 

 the unknown solution alternating with the known solutions are 

 drawn up into the tube and their lengths measured. After an 

 interval of time their lengths are again measured and the drop 



