ELASTICITY 329 



containing 120 cc. of the solution. It was found that dilute acids 

 very quickly led to a very marked diminution in cohesiveness, 

 allowing the di^op to fall off the rod and disperse, forming a cloudy 

 solution. More concentrated acid, however, maintains the cohe- 

 sion; in other words, with increasing concentration of acid the 

 cohesiveness of the gluten is first lowered, until it reaches a mini- 

 mum, and then raised until it regains its value in water. There 

 is no simple relationship between the concentration of an acid in 

 which cohesiveness is retained just sufficiently to hold the drop to 

 the rod and the concentration of hydrogen ions in these solutions 

 as indicated by their conductivities, although higher concentra- 

 tions of weak than of strong acids are as a rule required. Salts 

 lessen the power which acids and alkalies possess of weakening the 

 cohesion of gluten, and the concentration of salt required to nullify 

 the dispersive action of the various acids varies with the concen- 

 tration of acid in a very characteristic manner which is illus- 

 trated by numerous curves in Wood and Hardy's paper. These 

 authors interpret their results on the supposition that diminu- 

 tion in the cohesiveness of the gluten is brought about by the 

 appearance of electric double layers upon the surfaces of the 

 gluten particles. 



3. The Elasticity of Protein Solutions. — The elasticity of 

 protein solutions and jellies has been investigated by Rankine 

 (89), Rohloff and Shin jo (98), Reiger (94), Leick (58) and Hay- 

 craft (42). They find that even dilute solutions of gelatin resist 

 deformation to a slight extent, a fact which would appear to point 

 to the existence within these solutions of some species of molecular 

 (or ionic) structure such as that indicated in section 1. This 

 "form-elasticity" of gelatin solutions is diminished by heating. 



The form-elasticity of a freshly prepared gelatin jelly increases 

 with time until it attains a maximum. According to Haycraft, 

 gelatin jellies obey Hooke's law, that is to say, for moderate 

 (linear) strains the deformation which is produced is directly pro- 

 portional to the applied force, and the recovery, when the strain 

 is removed, is rapid and complete. 



According to Henderson and Brink (43) the compressibility of 

 gelatin solutions is somewhat less than the compressibility of water 

 and lower the greater the concentration of gelatin in the solution. 

 With varying compression the compressibility varies in much the 

 same way as the compressibilities of other solutions. 



