1. (a) U.S. Patent 2,890,253 Shell Development Co.
2. (b) Canadian Patent 576,172 Shell Development Co.
3. Belgian Patents 617,890 to 617,893 Petro-Tex Chemical Corp.
4. U.S. Patent 2,399,678.
5. Reidel, J. C., Oil and Gas Journal, p. 87, December 2, 1969.