Figure 5.—Oven for the calcination of bones, about 1870. “The operation is carried out in a rather high oven, such as shown.... The fresh bones are thrown in at the top of the oven, B. First, fuel in chamber F is lighted, and a certain quantity of bones is burnt on the grid D. When these bones are burning well, the oven is gradually filled with bones, and the combustion maintains itself without addition of other fuel. A circular gallery, C, surrounds the bottom of the oven and carries the products of combustion into the chimney, H. The calcined bones are taken out at the lower opening, G, by removing the bars of grid B.” (Translation of the description from Figuier, Merveilles de l’industrie, volume 3, 1874, page 537.)
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:
3 P 3 aq. pyrophosphoric acid