Physical Properties of Protoplasm 151 



section is the method employed for determining such properties as 

 solidity, hardness and tenacity or cohesiveness. All physical proper- 

 ties that have been enumerated are relative and it is hoped at a later 

 time to increase the accuracy of description by the selection of arbi- 

 trary standards. The usage of the terms employed in this paper is 

 based on the dissection of many widely different types of animal 

 and plant cells. 



Living matter occupies an intermediate position between true 

 solids and true liquids and has many of the properties of both as 

 well as properties peculiar to itself. It belongs to the class of colloids 

 known as emulsoids and exists in either a gel (hydrogel) or a sol 

 (hydrosol) state. 17 The term gel will be used to designate the amor- 

 phous semi-solid state and sol the apparently homogeneous liquid 

 state, of living substance. Protoplasmic sols usually appear as hazy 

 homogeneous liquids on account of the very minute size of the protein 

 aggregates that compose the solid phase. On the other hand pro- 

 toplasmic gels are characterized by the large size of the particles of 

 the solid phase which set to form the gel. Hence, living gels may 

 exhibit either a homogeneous or heterogeneous molar structure. 



It should now be clear that the term homogeneous is used in a 

 relative sense to describe the optical image and refers only to the 

 molar structure that can be brought out by the usual microscopical 

 powers and further that heterogeneity is the universal distinguishing 

 characteristic of colloidal sols and gels. In this connection it may be 

 noted that Pauli 18 states that the " unfixed" gel of gelatine is not 

 structured in the sense of being composed of threads, networks, 

 granules and vacuoles; it has the molar structure of a one-phase sys- 

 tem, 'which is precisely what is meant by the term homogeneous as 

 used in this paper; the molecular structure is unknown. The 

 present unsettled state of the problem of phase relations of colloidal 



17 For discussion of the classification of colloids see: No YES, A: 1905, Journal 

 of the American Chemical Society, 1905, xxvii, p. 85; OSTWALD, Wo.: 1907, 

 Zeitschrift fur Chemie und Industrie der Kolloide, 1907, i, p. 291; PERRIN, 

 J.: 1905, Journal de la chemie physique, iii, p. 50; FREUNDLICH and NEU- 

 MANN, 1908, Kolloid Zeitschrift iii, p. 80; VON WEIMARN, P. P.: ibid., 1908, iii, 

 p. 26. 



18 PAULI: Der Kolloidale Zustand und die Vorgange in der lebendigen Sub- 

 stanz, Braunschweig, 1902. 



