RHEOLOGY OF HYDROPHILIC POLYMER SOLUTIONS 143 the Agar Plate Method, which is a much more stringent test to meet. Naturally, the more test organisms employed, a more complete picture is obtained. As mentioned previously, no preservation tests are infallible but excep- tional success has been obtained by following these procedures. To summarize: 1. Cosmetic products can become contaminated with micro6rganisms by means of: (a) Raw materials (b) Ion exchangers (c) Unclean water filtering equipment (d) Contaminated plant equipment i.e., kettles, pipe lines, filling machines, etc. 2. Cosmetic products should be chemically preserved employing mixtures of suitable preservatives. 3. Laboratory preservation tests that have proved adequate have been outlined. 4. Cosmetic products should be subjected to these tests upon comple- tion of their development and periodically thereafter. REFERENCES (1) Cruickshank, G. A., and Braithwaite, D. G., fnd. Eng. Chem., 41,472 (1949). (2) Elsman, P. C., Kull, F. C., and Mayer, R. L., •. •7m. Pharm. •7ssoc. $ci. Ed., $8, 88(1949). THE RHEOLOGY OF NATURAL AND SYNTHETIC HYDROPHILIC POLYMER SOLUTIONS AS RELATED TO SUSPENDING ABILITY By R. J. M•¾•P. and L. Cou•* Presented November 20, 1958, New York City THE EFFECT OF gravity on suspensions or emulsions has long resulted in the general problem of sedimentation of solids and phase separation in emulsions. Because of an inability to cope with this handicap, many cosmetic products have failed at some stage of commerical development. The objective of this paper is to introduce a new mechanism for obtaining permanent suspension. This mechanism involves the rheology of the water phase in a static system. It will be shown that permanent sus- pension results when the water phase exhibits a critical minimum yield value irrespective of the apparent viscosity. * B. F. Goodrich Chemical Co., Avon Lake Development Center, Avon Lake, Ohio.

RHEOLOGY OF HYDROPHILIC POLYMER SOLUTIONS 143 the Agar Plate Method, which is a much more stringent test to meet. Naturally, the more test organisms employed, a more complete picture is obtained. As mentioned previously, no preservation tests are infallible but excep- tional success has been obtained by following these procedures. To summarize: 1. Cosmetic products can become contaminated with micro6rganisms by means of: (a) Raw materials (b) Ion exchangers (c) Unclean water filtering equipment (d) Contaminated plant equipment i.e., kettles, pipe lines, filling machines, etc. 2. Cosmetic products should be chemically preserved employing mixtures of suitable preservatives. 3. Laboratory preservation tests that have proved adequate have been outlined. 4. Cosmetic products should be subjected to these tests upon comple- tion of their development and periodically thereafter. REFERENCES (1) Cruickshank, G. A., and Braithwaite, D. G., fnd. Eng. Chem., 41,472 (1949). (2) Elsman, P. C., Kull, F. C., and Mayer, R. L., •. •7m. Pharm. •7ssoc. $ci. Ed., $8, 88(1949). THE RHEOLOGY OF NATURAL AND SYNTHETIC HYDROPHILIC POLYMER SOLUTIONS AS RELATED TO SUSPENDING ABILITY By R. J. M•¾•P. and L. Cou•* Presented November 20, 1958, New York City THE EFFECT OF gravity on suspensions or emulsions has long resulted in the general problem of sedimentation of solids and phase separation in emulsions. Because of an inability to cope with this handicap, many cosmetic products have failed at some stage of commerical development. The objective of this paper is to introduce a new mechanism for obtaining permanent suspension. This mechanism involves the rheology of the water phase in a static system. It will be shown that permanent sus- pension results when the water phase exhibits a critical minimum yield value irrespective of the apparent viscosity. * B. F. Goodrich Chemical Co., Avon Lake Development Center, Avon Lake, Ohio.