PHYSICO-CHEMICAL INTRODUCTION 31 



union of hydrogen and oxygen. The adsorptive power of colloids is 

 very great owing to their fine participate condition and enormous 

 surface, and this plays a great part in the chemistry and physics of 

 living cells. 



Most colloids are held back by very fine filters. Thus, the colloids 

 of blood-plasma can be separated by the use of a porcelain filter candle 

 which has been soaked in a solution of gelatin. The water and 

 salts can be pressed through such a filter. Colloids are indiffusible 

 through animal or vegetable membranes. The membranes themselves 

 are colloids, and, since colloids do not readily dissolve in colloids, 

 it is clear they will diffuse but little through each other. Crystalloids, 

 on the other hand, as we have seen, diffuse readily ; they are generally 

 soluble in colloids. This difference in property can be well demon- 

 strated by placing a stick of agar jelly (colloid) in some ammoniated 

 copper sulphate solution (crystalloid), and another in some Prussian 

 blue solution (colloid). It will be found that the blue copper solution 

 penetrates readily, the Prussian blue not at all. Colloids also appear 

 to influence physico-chemical processes but little. Crystalloids will 

 diffuse almost as readily from colloids as from water. So, too, chemical 

 processes take place in colloidal solution almost as if colloids were 

 absent. Advantage is taken of these properties in the body. A 

 crystalloid, when not linked or adsorbed to a colloid, will wander 

 freely and diffuse away from a cell; a colloid will remain where 

 it is formed. Thus, we find that the crystalloid dextrose is con- 

 verted in the liver into the colloid glycogen for storage purposes, 

 but to escape from the liver cell the glycogen is converted again 

 to the crystalloid dextrose. The osmotic pressure exerted by 

 colloids is very small or nil. It is believed that when absolutely pure 

 and free from traces of crystalloids colloids exert no osmotic pressure. 

 Also they depress the freezing-point of a solution but little. Increasing 

 the amount of egg albumin in water from 14 to 44 per cent, causes 

 but an alteration of freezing-point from 0-02 to 0-06 C. Since, also, 

 colloids ionize but little, they conduct electricity but little. On the 

 passage of an electric current through a colloidal solution, however, 

 the particles of most colloidal solutions tend to move in the electric field ; 

 cataphoresis, as the phenomenon is termed. This probably depends upon 

 the existence of a high surface tension in colloids. Surface tension may 

 be described as the force with which a fluid strives to reduce its free 

 surface to a minimum. When, therefore, we speak of the lowering 

 of the surface tension of a fluid, we mean that the force tending to 

 reduce its free surface is weakened, so that the free surface increases. 

 The formation of emulsions is due to such a lowering of the surface 

 tension. Water and oil will not mix, the oil floating on the surface 

 of the water, owing to the high surface tension of the oil. If, how- 

 ever, some soap be added to the water, the big oil drop is seen to break 

 down gradually into a number of smaller. 



Imbibition. Most of the organic colloids exhibit the property of 

 taking up fluid without chemical change. This is the phenomenon 

 of imbibition. For example, dry gelatin brought in contact with 



