EVOLUTION OF COLLOIDS 139 



slow spontaneous change when left to them- 

 selves. On the other hand, colloids are 

 known of great stability which are not easily 

 upset even by boiling their solutions, or by 

 saturating them with salt. 



These variations are a peculiarity of the 

 colloidal state of great importance in relation- 

 ship to the phenomena of life. 



A great many* colloids show two distinct 

 forms of arrangement of their multi-molecules, 

 known respectively as hydrosols and hydrogels. 

 Thus a solution of glue or gelatine at such a 

 temperature that it is fluid and mobile is a 

 hydrosol ; at a lower temperature it sets into 

 a solid jelly, and is then a hydrogel. The 

 clear fluid part of the blood, known as plasma, 

 or the clear uncooked fluid of the white of an 

 egg contains its nutrient protein substances 

 in the form of hydrosols. When either fluid 

 is heated nearly to the boiling point of water 

 it sets into a solid jelly and then forms a 

 hydrogel. The effect of temperature it will 

 be noticed, is in the opposite direction : 

 in the gelatine, which becomes fluid at the 

 higher temperature, while the egg-white and 

 blood-plasma set solid or coagulate at the 

 higher temperature. There is also this import- 

 ant difference that the process is reversible 

 in the case of the gelatine, which is hence 



