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- 
