98 



DISPERSE SYSTEMS 



Laminaria balanced with cork and wire on zero day. On the first day it 

 floats, on the second it is as on zero day, and on the third day it sinks. If 

 more cork is now added so that the system just floats again, the laminaria 

 will sink again on the flfth day. 



If IV = wt. of cork and wire, v = vol. of cork and wire. 

 Then on zero day, w + 2-64 = v + 1"81 (/."., density = 1) 



.■ . v= w -\- 0-83. 

 Then on any day (say 10th) 



' Density of whole system 



. wt. m; + 20-31 



IS 



vol. ~ -y + 19-40 

 _w + 20-31 

 ~ w + 20-23 



which is > 1. 



Hence on the lOth day the laminaria will have sunk if just balanced on the 

 zero day. 



It will be seen that at first the density of the system decreases 



Kl(i. -^-d 



-(IvloniftcT for determining tlie swelling pressure of colloids. 



slightly, and then rapidly increases — the system sinking in water. 

 The primary decrease, occurring when the macropores are filling 

 and the seaweed swells, is difficult to explain and is not relevant to 

 this discussion. During the period of increasing density, water is 

 being packed into the micropores in the interior of the gel. This 

 water is " bound " and can only be driven off by the application 

 of suction and heat. Many similar experiments have been devised 

 to show the same phenomenon. In Fig. 22 is an apparatus 

 designed to measure the swelling pressure of seeds or of powdered 

 gelatin. (See also Part II., p. 537.) 



The " boimd " portion of the imbibed water which is held so 

 fiercely, and which may be so increased in density that it 

 occupies about 75 per cent, of its bulk at atiuospheric pressure, 

 has no aj^preeiable vapour pressure at ordinary temperatures 

 (Gortncr). It will, therefore, freeze with great difficulty, and if it 



