INTRODUCTION 9 



teristics of protoplasm. To understand the activi- 

 ties of protoplasm we must therefore know something 

 of the physics and chemistry of colloids. 



Colloids (from colla = glue) do not diffuse, or 

 diffuse very slowly, through animal membranes ; in 

 this respect they differ from crystalloids, which 

 diffuse comparatively rapidly through animal mem- 

 branes. Wolfgang Ostwald recognized two sorts 

 of colloids : (1) suspension colloids, which are mix- 

 tures of solid and liquid phases, are non- viscous, 

 and easily coagulated by salts, e.g. a mixture of 

 finely divided metal and water; and (2) emulsion 

 colloids, which are composed of two liquid phases, 

 are viscous, and coagulated by salts with difficulty. 

 Protoplasm is rich in emulsion colloids; these may 

 exist as liquid sols, or more solid gels. In either 

 case they consist of fine colloidal particles. Accord- 

 ing to another classification colloids may be separated 

 into reversible and irreversible; the former may 

 change from the sol to the gel state and back again, 

 but the latter are unable to do this. Protoplasm is 

 a reversible colloid, and many cellular structures 

 appear to originate through the gelation of liquid 

 colloids. Since protoplasm is a sol or gel due to 

 water, it is a hydrosol or hydrogel, and because 

 of its water content is said to be hydrophylic. It 

 contains crystalloids and its chemical reactions take 

 place in a dilute solution of electrolytes ; these are 

 substances which dissociate, at least in part, into 

 their constituent ions when in solution, and the ions 

 are electrically charged. For example, NaCl disso- 



