According to Gilbert the primary source of matter is the interior of the earth, where exhalations and "spiritus" arise from the bowels of the earth and condense in the earth's veins.[49] If the condensations, or humors, are homogeneous, they constitute the "materia prima" of metals.[50] From this "materia prima," various metals may be produced,[51] according to the particular humor and the specificating nature of the place of condensation.[52] The purest condensation is iron: "In iron is earth in its true and genuine nature."[53] In other metals, we have instead of earth, "condensed and fixed salts, which are efflorescences of the earth."[54] If the condensed exhalation is mixed in the vein with foreign earths already present, it forms ores that must be smelted to free the original metal from dross by fire.[55] If these exhalations should happen to pass into the open air, instead of being condensed in the earth, they may return to the earth in a (meteoric) shower of iron.[56] [49] M: pp. 21, 34, 35, 36, 45. [50] M: pp. 35, 36, 38, 69; see, however, pp. 42-43: "Iron ore, therefore, as also manufactured iron, is a metal slightly different from the homogenic telluric body because of the metallic humor it has imbibed ..." [51] M: pp. 19, 34, 36, 37, 42, 69. [52] M: pp. 35, 36, 37, 38. [53] M: pp. 38, 63, 69, 84; on p. 34 he says that iron is "more truly the child of the earth than any other metal"; it is the hardest because of "the strong concretion of the more earthy substance." [54] M: pp. 21, 35, 37, 38. [55] M: pp. 35, 63. [56] M: pp. 45, 46. Gilbert was indeed writing a new physiology, both in the ancient sense of the word and the modern. The process of the formation of metals had many biological overtones, for it was a kind of metallic epigenesis.[57] "Within the globe are hidden the principles of metals and stones, as at the earth's surface are hidden the principles of herbs and plants."[58] In all cases, the "spiritus" acts as semen and blood that inform and feed the proper womb in the generation of animals.[59] "The brother uterine of iron,"[60] the loadstone, is formed in this manner. As the embryo of a certain species is the result of the specificating nature of the womb in which the generic seed has been placed, so the kind of metal is the result of a certain humor condensing in a particular vein in the body of the earth. [57] Gilbert's terminology strongly suggests that he was familiar with alchemical literature, as well as that of medical chemistry. He has been credited as being highly skilled in chemistry. See Sir Walter Langdon-Brown, "William Gilbert: his place in the medical world," _Nature_, vol. 154, pp. 136-139, 1944. [58] _Ibid._, p. 37. [59] M: pp. 35, 36, 53, 59. See also Galen, _op. cit._ (footnote 15) bk. 2, ch. 3. [60] M: pp. 16, 59.