MINERAL CONSTITUENTS 131 



or through them quite readily. Because of this lack of power to 

 diffuse, colloids possess very little osmotic pressure. Pfell'er gives 

 the osmotic pressure of a one per cent solution of sugar as equiva- 

 lent to a column of mercury 51.8 cm. high, while that of a one per 



Dialysis and Diffusion of Colloids and Crystalloids 



Substances 



Crystalloids 



Sodium chloride 1 .00 



Ammonia 0.85 



Alcohol j 0.47 2.0 



Glucose. . , 0.36 3.0 



1.0 

 0.0 



Cane sugar. 



0.47 



Average ; 0.63 



3.0 



1.92 



cent of gum is only (>.!) cm. From the above table it will be seen that 

 crystalloids diffuse over ten times as rapidly as colloids. 



(e) Freezing and lioiling Point*. The lowering of the free/ ing 

 and boiling points by crystalloids such as common salt in solution 

 is familiar to every one. The change in these is in proportion to 

 the amount dissolved. Colloids have very little effect. Forty-four 

 grams of protein dissolved in 100 grams of water lowered the free/- 

 ing point only 0.06 C. 7 



(f) Electrical Behavior. Colloids are poor conductors of elec- 

 tricity as compared with crystalloids, and their conductivity de- 

 creases with the amount of colloid in the disperse medium. Any 

 substance in contact with water and many other liquids acquires an 

 electric charge. ^lost substances become negatively charged in con- 

 tact with water. The charge can be varied and even reversed by 

 electrolytes and may even become zero at certain suitable concen- 

 trations. If a current of electricity is passed into a colloidal solu- 

 tion, the particles migrate to one pole or the other. If they miirrate 

 to the negative pole (cathode) they are positive, and if toward the 

 positive pole (anode) they are negative. The colloidal condition 



