Figure 6.—An advertisement with view of plant for manufacturing superphosphate about 1867. (From E. T. Freedley, Philadelphia and its Manufacturers in 1867, page 288.)
Figure 7.—Florida hard-rock phosphate mining. (From Carroll D. Wright, The Phosphate Industry of the United States, sixth special report of the Commissioner of Labor, Government Printing Office, Washington, 1893, plate facing page 43.)
Graham had come to this understanding of the phosphoric acids through his previous studies of “Alcoates, definite compounds of Salts and Alcohol analogous to the Hydrates” (1831). Liebig started from analogies he saw with certain organic acids when he formulated the phosphoric acids with a constant proportion of water (aq.) and varying proportions of “phosphoric acid” (P) as follows:
2 P 3 aq. phosphoric acid