COLLOIDS AND COLLOIDAL SOLUTIONS 387 



Properties of Gels. 



The gels formed by silicic acid, gelatin, agar, etc., which may contain 

 as much as 90 per cent, of water, possess some of the properties of solids. 

 They may be put into two groups the rigid or nearly non-elastic 

 gel like silicic acid and the elastic gel like gelatin, collodion, etc. 



(1) Behaviour to Water. 



(a) The rigid gel of silicic acid is translucent ; on exposure to the 

 air it loses water, becoming opaque, and with loss of more water it 

 again becomes clear. The amount of water present in the solid 

 material corresponds with the tension of aqueous vapour, the ratio 

 of the constituents, silicic acid and water, changing continuously. 

 No definite hydrate is formed such as occurs with crystals containing 

 water of crystallisation. The formation of many siliceous minerals 

 may be accounted for in this way. The elastic gel, like gelatin, which 

 is reversible, also behaves in a similar manner to water ; the amount 

 of water present in it depends on the tension of aqueous vapour. 



(b) Gelatin behaves differently when immersed in water ; it swells 

 and much more water is taken up ; it is given off again on exposure to 

 the air. This absorption of water is of great physiological importance. 



(c) Though an absorption of water and swelling take place when 

 an elastic gel is put into water, the actual volume of the gel and water 

 is less than the total volume of the two substances. There is compres- 

 sion of the water. 



The decrease in volume is demonstrated by Hatschek by placing 

 a known weight of gelatin in a pycnometer, filling it with water, and 

 immersing the vessel in water. When the gelatin has swollen, the 

 vessel is taken out of the water, dried and weighed. There is an in- 

 crease in weight which shows that water has entered the vessel. To 

 compress water to an extent corresponding to 2 per cent, of the original 

 volume requires 400 atmospheres. 



(d) Heat is liberated during the swelling of the gels. It has been 

 measured and found to vary from 5 to 10 gm. calories per gm. of gel. 



(e) The total volume decreases, but the gel in water swells. 

 This increase in volume has been measured and it has been found that 

 against a pressure of 42 atmospheres a gel will swell by 1 6 per cent, 

 of its volume ; against a pressure of I atmosphere the increase in 

 volume is 330 per cent. 



From this it can be calculated that I gm. of gel on swelling will 

 lift I kilo, to a height of 3-3 cm. 



(2) The elasticity, the optical constants and thermal expansion of 

 gels differentiate them from both liquids and solids. 



13* 



