108 THE PHILOSOPHY OF BIOLOGY 



was paid to them by the physiologists, so that Hfe was 

 called " the chemistry of the colloids," just as after the 

 investigation of the enzymes it was called the 

 " chemistr}^ of the enzymes," and when the discovery 

 of the relative abundance of phosphorus in cell-nuclei 

 and in the brain was discovered, it was called the 

 " chemistry of phosphorus." Colloids (e.g. glue) are 

 substances that do not readily diffuse through certain 

 membranes, in opposition to crystalloids {e.g. solution 

 of common salt) which do readily so diffuse. They 

 form solutions which easily gelatinise reversibly, that 

 is, can become liquid again (glue) ; or coagulate 

 irreversibly, that is, cannot become liquid again 

 (albumen) ; which have no definite saturation point ; 

 which have a low osmotic pressure (and derived pro- 

 perties), etc. ; and the molecules of which are com- 

 pound ones consisting of combinations of the molecules 

 of the substance with the molecules of the solvent, 

 or with each other, that is, they are molecular aggre- 

 gates. 



Colloids pass insensibly into crystalloids on the one 

 hand and into coarse suspensions (water shaken up 

 with fine mud, for instance) on the other. We may 

 replace the concept of a colloid by those of " sus- 

 pensoids " and " emulsoids." A suspensoid is a liquid 

 containing particles in a fine state of division — if the 

 division is that into the separate molecules we have a 

 solution, if into large aggregates of molecules we have 

 a suspension. If the substance in the liquid is itself 

 liquid, the whole is called an emulsoid. On the one 

 hand this approaches to a mixture of oil in soap and 

 water — an emulsion — and on the other hand to such 

 a mixture as chloroform shaken up with water, when 

 the drops of chloroform readily join together so that 

 two layers of liquid (chloroform and water) form. 



