108 THE PHILOSOPHY OF BIOLOGY 



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

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

 investigation of the enzymes it was called the 

 " chemistry 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. 



