Heistand, Robert N. (1976). "The Fischer Assay, standard method?"(PDF). San Francisco: Symposium on oil shale, tar sands, and related materials—production and utilization of synfuels. Archived from the original(PDF) on 2012-02-11. Retrieved 2008-08-18.
Heistand, Robert N. (1976). "The Fischer Assay, standard method?"(PDF). San Francisco: Symposium on oil shale, tar sands, and related materials—production and utilization of synfuels. Archived from the original(PDF) on 2012-02-11. Retrieved 2008-08-18.