66 



COLLOIDS IN BIOLOGY AND MEDICINE 



solutions. An excellent example of the latter is described by C. 

 LUDWIG. He hung a well-dried animal bladder in concentrated salt 

 solution. The bladder swelled, taking up a dilute salt solution and 

 common salt crystallized out in the remainder. Amphibia, e.g., 

 frogs, may lose one-fourth of their body weight upon drying as has 

 been shown by E. OvERTON.* 3 Although they contain about 80 

 per cent water, the osmotic pressure of the blood almost doubles. 

 The explanation of this is that only a portion is water of solution, 

 the remainder is water of swelling. Upon drying the water of swell- 

 ing is more strongly retained than the water of solution. 



Swelling exhibits manifestations of energy to no less a degree than 

 osmotic pressure. I shall give several examples taken from WOLF- 

 GANG OSTWALD'S "Grundriss." According to the investigations of 

 the plant physiologist, HALES, swelling peas were able to lift the 

 cover of an iron pot weighted with 83.5 kilograms. H. RODEWALD 

 found that it requires 2523 atmospheres pressure to overcome the 

 swelling pressure of starch. J. REINKE* determined the swelling 

 pressure of laminaria, a sea weed. Some of his data quoted from 

 H. FREUNDLICH give us an idea of the enormous pressures, changes 

 in volume and amounts of water taken up when swelling occurs 

 and of the pressures required for dehydration. Ten layers of dried 

 laminaria scales each 0.1 mm. thick and 50 mm. 2 were placed in the 

 apparatus. 



We obtain a fair idea as to the general course of swelling by ob- 

 serving a sheet of gelatin. Dry gelatin takes up one-third of its weight 

 of water from a moisture-saturated atmosphere at room temperature, 

 in order to reach a condition of equilibrium. If this sheet is then 

 placed in a dish of cold running water it absorbs from it 10 times its 

 dry weight of water in order again to reach a condition of equilibrium. 

 In dry air the water evaporates and shrinking occurs. The experi- 

 ment may be repeated as often as desired with the same result. On 

 this account substances of this group are termed elastic gels. 



Coagulated albumin, e.g., boiled fibrin, behaves differently. If it 

 is air-dried a horny residue remains which, though it takes up 

 some water, or swells when it is placed in water, never again 



