THE STRUCTURE OF PROTOPLASM 259 



framework out of little solid matter. Gels of iron oxide and 

 calcium germanate, containing but 0.1 per cent of solid matter 

 and, therefore, 99.9 per cent of water, are examples of thixotropic 

 systems. Thin soap solutions, egg white, and the jellyfish are 

 examples of highly elastic systems of low sohd content. Proto- 

 plasm is both thixotropic and highly elastic. 



The brush heap alone satisfies elastic and rigid qualities in 

 systems of high water content, but the thixotropy (of gels which 

 are readily broken down by mechanical disturbance and equally 

 readily build themselves up again) and the fluidity of elastic 

 systems require the additional quality of loose bonds between 

 the fibrous molecules so as to permit ready readjustment. 



Evidence that loose bonds may occur along the fibers of 

 elastic systems {e.g., at the points of contact of the imaginary 

 linear molecules in Fig. 126) and thus allow for a mechanical 

 interpretation of fluidity is to be had from examples of tautomeric 

 slips. One such is the change in ionization (and therefore in 

 bonds) known to occur in protein solutions with change m pH. 

 (This instance is less applicable to the problem in hand, as 

 there are apparently no changes in pH to account for readjust- 

 ment in structure when an elastic solution flows or when a 

 thixotropic gel solvates or gelates.) Another example is the 

 case of internal salt formation in proteins where amphoteric chain 

 molecules of the type of amino acids are converted into rings 

 by the acid and basic radicals mutually satisfying each other; 

 in this manner, the active linear molecule of glycine becomes an 

 inactive ring (page 485). Still another instance of a tautomeric 

 slip which illustrates the ease with which bonds may be shifted 

 is that from the keto to the enol form of glyoxal and back again: 



^O /OH 



R— C— CH ^ R— C=C=0 



^O 



If we turn from these more complex instances of tautomeric 

 shifts to the very simple case of water, we have an excellent 

 example of structural continuity, due to polarity, in a liquid 

 that flows freely owing to constant and ready tautomeric slipping 

 between its molecules. The water molecule is polar; i.e., it 

 has an electric moment (Fig. 138). It possesses, therefore, the 



