252 The Structure of Protoplasm 



is not a surface or interfacial phenomenon, but a property of the gel 

 as a whole, the basic mechanism of which lies in structural con- 

 tinuity. 



There then came into colloidal chemistry the stimulating concept 

 of a brush-heap of molecular or colloidal fibers. My earliest^^ inter- 

 pretation of the mechanism of elasticity in protoplasm was that of a 

 brush-heap, a haphazard distribution of molecular rods (Fig. 2, p. 

 4). The concept holds well for elasticity, but fails when other 

 qualities of protoplasm and jellies in general are considered. The 

 folded protein molecule (Fig. 4, p. 32) best meets all requirements, 

 and is the molecular form now commonly ascribed to elastic organic 

 substances and tissues such as albumen, wool, hair, and muscle. 



A folded protein chain is the basic mechanism of both the 

 elastic and contractile qualities of protoplasm and certain other 

 organic substances. I shall reserve further discussion of it for the 

 section on contractility, to follow. 



Though the folded chain is the most likely molecular form 

 responsible for the elasticity of gels, there are other possibilities of 

 which the helix is one. The molecular structure of rubber is thought 

 to be that of a modified helix. 



That a hydrocarbon could be elastic seemed so unlikely that when 

 it was suggested at the inaugural meeting of the Society of Rheology 

 that those structural features responsible for the anomalous flow of 

 gelatin, soaps, and like substances are also responsible for their 

 non-Newtonian behavior, objection was raised on the grounds that 

 hydrocarbon oils were found to be anomalous by Hardy, and hydro- 

 carbons cannot be elastic. Since then, artificial rubber of high 

 elastic qualities has been made of polymerized hydrocarbons. 



An interpretation of the molecular structure of hydrocarbons 

 such as will account for their elasticity, has been made by Mark^-, 

 Mack^"', and Meyer.^ ' They assumed the rubber molecule to be a 

 long chain, or molecular fiber fashioned as a helical spring, which 

 uncoils on stretching. 



In order to account for an 800 per cent stretch in rubber, Mack 

 devised a mechanism wherein the axis of one molecule is perpen- 



" Amer. Naturalist, 60, 124. 1926. 



''Mark, H., Physical Chemistry of High Polymeric Systems, N. Y. 1940. (See 

 also Kautschuk, 6:2. 1930). 



'Jour. Amer. Chem. Soc, 56:2757. 1934. 



" Meyer, K., Die Hochpolymeren Verbindungen, Leipzig, 1940 (See also Chem. 

 Revs., 25:137. 1939). 



