2 PRINCIPLES OF STRUCTURE 5I 



water in the form of solid skins (Devaux, 1935; Gorter and co- 

 workers, 1955; JoLY, 1948). The structure of such films is not yet 

 known in all its details. Molecules of the polypeptide chain type 

 (Fig. 87c, p. 132) do not stand erect but lie flat on the surface. As a 

 result of their amphoteric nature, their spreading surface is not con- 

 stant but depends on p^^. It is important to note that, judging from 

 their surface activity, not only the skeletal proteins but also the re- 

 serve proteins are hydrophobic to a considerable degree (Bull, 1947). 

 The surface structures described in this section are brought about, 

 not by primary valencies, but merely by cohesion forces. Consequently, 

 the relative positions of the atoms are not fixed like those in a main 

 valency lattice; a certain mobility exists, of which indications were 

 already found in the ease with which molecule lattices are split and 

 deformed. In surface films, however, the attractive forces are still 

 less pronounced. The molecules in a film containing fatty acids, for 

 instance, are free to rotate about their axis. We might say that surface 

 films are in a state intermediate between the amorphous liquids and 

 the solid bodies with their well-defined regular structure. 



d. Liquid Crystals 



Mesophases . At one time "liquid crystals" played a great part in the 

 discussion of protoplasm structure. Lehmann (1917) went so far as 

 to attribute life to these remiarkable structures. We know now, 

 however, that the unusual properties of "flowing" crystals which, 

 on account of their striking birefringence, are perhaps better denoted 

 as anisotropic liquids^ are by no means as enigmatic as was formerly 

 believed. For, the structure of liquid crystals is similar to that of the 

 surface films of fatty acids on water. It is usually a matter of chain 

 molecules in parallel alignment, which are free to move relatively to 

 each other in the direction of their axis and to rotate about this axis. 

 However, the orientation in the surface films is restricted to a small 

 number of monolayers or even to a single monolayer only, whereas 

 the liquid crystals contain oriented structures of microscopic dimen- 

 sions (deformable crystals, drops, etc.) 



The best starting point for a correct understanding of the structure of 

 crystalline liquids is the molecule- or chain-lattice represented in Fig. 52 

 (p. 35). In these lattices the molecules are immovable; the substance is 

 in the crystalline solid state. If, now, heat is applied to the lattice, the 



