Sols and Gels — Relation to Protoplasmic Structure 97 



to ultrasonics, which were too weak to cause cavitation but strong 

 enough to cause coagulation, it solidified in 15 seconds, whereas it 

 remained liquid for 17 minutes if left to itself; it took 30 seconds to 

 cause the rheopectic solidification by tapping.'''' A dilatant suspension 

 is kept liquid by ultrasonics; it does not show the characteristic 

 increase in mechanical resistance, when treated with a glass rod.-^^ 

 This effect is not due to cavitation. 



Finally, there is still a third effect, which appears with very weak 

 ultrasonics: Rod- or plate-like particles of a suspension are oriented 

 with their long axes vertical to the direction in which the energy of 

 the waves flows. •''^ 



FOOTNOTES 



'Verworn, M., Allgemeine Physiologie, Jena, (1895); Wilson, E. B., The Cell 

 in Development and Inheritance, New York, (1897). 



' For a general survey of thixotropy, see H. Freundlich, Thixotropy, Actualites 

 scientifiques et industrielles, No. 267, Paris, Hermann et Cie., (1935). 



'Freundlich, H., and Roder, H. L., Trans. Farad. Soc, 1938, 34. 308; R6der, 

 H. L., Rheology of Suspensions, Ph. D. Thesis, Amsterdam, 1939. 



^ Sameshima, I., Bl. chem. Soc, Japan, 1926, 1, 255. 



■ Laing, M. E., Jour. Phys. Chem., 1924, 28, 673. 



'■ Freundlich, H., and Seifriz, W., Z. phijs. Chem., 1923, 104, 233. 



• Szegvari, A., and Schalek, E., Kolloid-Z., 1925, 32, 318; 33, 326; and many later 

 articles, mostly by Freundlich and co-workers; for references, see Freundlich, 

 H., Kolloid-Z., 1928, 46, 289, and FreundUch, H., Thixotropy, 1935 (see footnote 

 2). 



'Freundlich, H., Schmidt, O., and Lindau, G., Z. phys. Chem., Bodenstein- 

 iestband 1931, 333; Kolloid Beihejte, 1932, 36, 43; Hauser, E. A., and Reed, C. E., 

 Jour. Phys. Chem., 1937, 41, 911. 



' Edsall, J. T., and Mehl, J. W., Jour. Biol. Chem., 1940, 133, 409; Mirsky, A. E., 

 Cold Spring Harbor Symposia on Quantitative Biology, 1938, 6, 150. 



'"LaufTer, A. M., and Stanley, W. M., Kolloid-Z., 1940, 91, 62 (no mention is 

 made in this paper of thixotropy, but there is anomalous viscosity). 



" Zocher, H., and Albu, H. W., Kolloid-Z., 1928, 46, 27. 



'•' Zocher, H., and Albu, H. W., Kolloid-Z., 1928, 46, 33. 



'Freundlich, H., and Abramson, H. A., Z. phys. Chem., 1927, 131. 278; Heller. 

 W., Compt. rend., 1936, 202, 1507. 



" Freundlich, H., and JuUusburger, F., Trans. Farad. Soc, 1934, 30, 333; 

 Freundlich, H., and Jones, A. D., Jour. phys. Chem., 1936, 40, 1217. 



'' Reckhnghausen, H. V., Kolloid-Z., 1932, 60, 34. 

 '"Green, H., hid. and Eng. Chem., 1923, 15. 122. 

 ''Reynolds, O., Phil. Mag. (5), 1885, 20, 469; Nature, 1886, 33, 429. 

 '' Olze, A., and Daniel, F. K. quoted by Freundlich, H., and Roder, H. L., 

 Trans. Farad. Soc, 1938, 34, 308. 



" Freundlich, H., and Jones, A. D., Jour. Phys. Chem., 1936, 40, 1217. 



'"Freundlich, H., and S511ner, K., Kolloid-Z., 1928, 45, 348. 



"Heller, W., Compt. rend., 1936, 202, 61; Jonr. Phys. Chem., in press. 



'^ Heller, W., and Quimfe, G., Compt. rend.. 1937, 205, 857; Heller, W., and 

 Vassy, E., Co-nipt, rend., 1939, 208, 812, Jour. phys. Chem., in press. 



