C. SWMLL1XG PROCESSES 



169. Arrange a series of test-tubes of uniform diameter and 

 place in each tube i gram of granulated gelatin. Determine the rela- 

 tive degrees of swelling undergone by the gelatine in the following 

 solutions. The gelatine is mixed with 10 cc. of the solution and 

 allowed to stand until the height of the swollen gelatine in the tube 

 is constant. 



a. distilled water. 



b. n/5, n/io, 11/20, 11/40 HC1. 



c. 11/5, n/io, 11/20, 11/40 NaOH. 



d. n/5 NaCl, 11/5 Na 2 SO 4 . 



e. n/20 HC1 + n/io NaCl; 11/20 NaOH -f n/io NaCl. 



Note the influence (a) acid, salt, and alkali acting alone; (b) opti- 

 mum concentration for swelling; (c) action of acid and alkali in 

 presence of neutral salt. Antagonisms of this latter kind are of great 

 physiological importance. 



D. OSMOTIC PRESSURE OF COLLOIDS. 



Direct determinations of osmotic pressure are difficult to make with 

 crystalloid substances because of the difficulty of preparing satis- 

 factory semi-permeable membranes. On the other hand, semi- 

 permeable membranes for colloids are easily prepared, so that, 

 although the osmotic pressure is low, there is no difficulty in measur- 

 ing it directly. The osmotic pressure of colloids varies with their 

 "state of aggregation", and this varies with the concentration of the 

 electrolytes present in solution along with the colloid, and also with 

 several other conditions: as (i) rate of admixture of electrolyte; 

 (2) degree of mechanical agitation to which the solution has pre- 

 viously been exposed; (3) the temperature; and (4) in general, the 

 lapse of time and the nature of the previous history of the colloid 

 (method of preparation, etc. ). 



i/o. A simple and efficient osmometer is made as follows: make 

 a collodion membrane of the shape and capacity of a 50 cc. round- 

 bottomed flask: this is done as follows: Pour a moderate quantity 

 of the 10% collodion solution (in equal parts alcohol and ether) into 

 a 50 cc. flask ; invert the flask and turn till an even layer of solution 

 is formed on its wails; pour back the surplus solution into the bottle; 

 blow a current of air into the flask through a glass tube; then add 

 some warm water and change this two or three times. The mem- 

 brane is then ready to remove from the flask ; removal is facilitated 



4.3 



