26 PHYSICAL CHEMISTRY IN MEDICINE. 



are divisible into two groups the electrolytes, which in 

 (aqueous) solution conduct the electric current, and the 

 non-electrolytes, which do not. To the first class belong 

 the salts, acids, and bases ; to the latter most of . the 

 organic substances, such as urea and sugar. 



Between the two great groups of colloids and crystal- 

 loids there exists no sharp line, for we are acquainted 

 with " half -colloids, " which stand midway between these 

 extremes. But because of the typical differences existing 

 between the extremes of the whole series, differences 

 which all substances show more or less perfectly, this 

 division into colloids and crystalloids is nevertheless of 

 great value. 



The colloids exist in two states, a liquid and a solid 

 state. In the liquid state they are known as sols (GRA- 

 HAM), in contrast to the solid state, in which they appear 

 as dry, swollen, coagulated (through heat or ferments), 

 or precipitated (for example, through electrolytes) masses, 

 which are known as gels. A question that arises at once 

 is, Do the colloids of living matter exist in the sol or in 

 the gel state ? 



Protoplasm possesses properties which are character- 

 istic, generally speaking, of both solid and liquid sub- 

 stances. This peculiarity of living matter has given rise 

 to great discussions between the believers in the solid 

 and those in the fluid state of aggregation of protoplasm. 

 The ability to stand alone in other words, a relative 

 independence in form, which often expresses itself in the 

 existence of characteristic cell forms corresponds with 

 the properties of the solid state, while an argument for 

 the liquid state of protoplasm is readily found in the 

 general and necessary condition that chemical reactions 



