Indeed, roughly the first third of De fontibus is dedicated to two subjects: ‘Descriptions of the four better known fossil salts’—vitriol, alum, saltpetre, sea salt—and a lengthy discussion on ‘Veins of Iron’ and pyrites, also known as ‘fool's gold’. Whereas the latter is the subject of the second and longest chapter in the book, mineral waters are first discussed only in the fourth chapter. Lister had a long-standing interest in pyrites, going back to 1670s and to an unpublished manuscript in which he described pyrites as ‘ironstone marcasites’, which were ‘nothing else but a body of iron disguised under a vitriolic varnish’. The exploration of pyrites in a book on ‘Healing Springs’ can be understood only in reference to Lister's cosmology, in which pyrites and related chymical processes had key explanatory roles.
Lister asserted that salts form or ‘grow into crystals’ either quickly through dehydration or in a slower way, a process he likened to the germination of plants. His vitalist reasoning was based on an analogy between pyrites and limestone: ‘A ferrous vein is of course the parent of green vitriol, and limestone of salt of lime.’ By ‘ferrous vein’ Lister meant pyrites. While vitriol develops from pyrites, salt of lime (nitre of lime) grows out of limestone. On exposure to moist air, pyrites (iron sulphide, FeS2) undergoes a spectacular change, turning into ‘green vitriol’ (iron(II) sulphate, FeSO4) a green salt widely used in ink manufacture and wool dyeing since the Middle Ages.
Extending the analogy, Lister perceived the visibly striking transformation of pyrites into green vitriol, and of limestone into nitre of lime, as comparable processes, which he associated with the way in which ‘plants germinate’, gradually maturing and turning green in the presence of air. Like vitriol, limestone salt could only result from the exposure of limestone to air: ‘nitre of lime is produced in one and the same way as vitriol.’ He derived this assumption analogically and empirically. Nitre of lime was produced by the exposure of limestone to air because ‘where there is nitre of lime, there is always limestone to be found.’ Lister was probably observing the formation of saltpetre on walls that had been whitened by limestone.
On the basis of what he considered as a pioneering analogical explanation (by analogy to pyrites), Lister thought he was ‘the first … to give the shape and description of this lesser known salt [of limestone].’ He specified that the salt would not form when limestone was steeped in water. Thus limestone salt—like vitriol and plants—developed only in the presence of air through a slow and gradual maturation process. The explanation was modelled on the chymical behaviour of pyrites
