1920] SEI FRIZ— PROTOPLASM 363 



increase in consistency is not too great, a firm but resilient and 

 cohesive jelly results. Between these two conditions the exact 

 colloidal state cannot be readily determined. The general term 

 gel is consequently applicable. 4 



Scale of viscosity values 



■ All sciences suffer from a confusion in nomenclature, but per- 

 haps biology more than any other. For example, " viscosity 7 ' to 

 the physicist is a specific and accurately measurable property of 

 matter. All fluid substances have their mathematically deter- 

 mined coefficients of viscosity in physics, but to the biologist 

 viscosity has a meaning no more definite than that the substance 

 described has a density somewhere between that of water and of 

 steel. This vagueness, however, is not altogether the biologist's 

 fault. 



Protoplasm cannot be collected in sufficient quantity, nor 

 would it remain normal long enough to determine its viscosity, as 

 the physicist would determine that of a liquid, by running it 

 through a viscosimeter; consequently, any attempt to specify the 

 consistency of protoplasm will be more or less influenced by per- 

 sonal opinion. The personal element, however, can be restricted 

 to observations, and not be allowed also to determine the conno- 

 tation of loose expressions, such as " non- viscous ' ' (an impossible 

 term) and "very viscid sol/' which may have quite a different 

 meaning from that conveyed to the reader. 



In a previous paper (23) a scale of viscosity was devised so 

 that an expression such as " decidedly viscous" would hold a 

 definite place, and thus convey some idea of approximately how 

 viscous a " decidedly viscous" substance is. I shall retain this 



4 1 have heard from the younger colloid chemists that true gels are obtainable 

 only from emulsoids; therefore the coagulation of a suspensoid is not a gel; that is, 

 a gel is not a coagulum; it is a jelly. Some gels (jellies) are reversible and some not. 

 To exclude coagulation as a word descriptive of the process of hardening of emulsions 

 would be altogether too radical a departure in biology, where we constantly speak 

 of the coagulation of blood, of albumin, and like emulsions. I shall, with Bechhold, 

 consider as coagulations such processes as cause an irreversible change, whether from 

 emulsions or suspensoids. It would be a great service to biologists, and undoubtedly 



mis 



even though arbitrarily and but tentatively. 



■■ 



