he is more specific about the origin of earths through fire. The concluding statements of sections 54 and 69 are of particular interest, since they express his belief that fire was the causative agent in the formation of certain earths. The final statement of Theophrastus in this present section is of special significance in the history of theories of combustion, since it is really a root idea underlying most of the theories promulgated before the time of Lavoisier. The famous and erroneous phlogiston theory of Becher and Stahl, for example, which was so widely accepted in the eighteenth century, embodied the same idea as a fundamental principle.
51. orpiment, realgar. It is important to note that these mineral substances are nearly always mentioned together by ancient writers, since this helps to identify them as the two native sulfides of arsenic. Theophrastus has previously mentioned them together in section 40 and again in section 50. Both Dioscorides370 and Pliny371 mention them in successive sections. The names arrhenicum and sandaraca that Pliny uses are obviously mere transliterations of the Greek names used by Theophrastus. Orpiment, native yellow arsenic sulfide (As2S3), and realgar, native orange-red arsenic sulfide (As4S4), nearly always occur together and are often found intermingled in the same small specimen. Pliny alludes to the mixed form of the two minerals, and Dioscorides states that they are found in the same mines. This peculiar mode of occurrence is sufficient to explain why they are always discussed together by ancient authors. Since the Leyden Papyrus X372 and other early technical works mention the use of one or the other of these minerals for whitening copper, it is almost certain that they were arsenic compounds, and the various descriptions of their physiological effects supply further evidence. The descriptions of their physical appearance given by Dioscorides and by Pliny are adequate to identify them as these particular arsenic minerals.
A few specimens of ancient orpiment and realgar have been found and positively identified as such by chemical tests. During the excavations at Corinth in 1926 Shear found a pottery vessel
37»V, 120, 121 (Wellmann ed., V, 104, 105). 371 XXXIV, i77> 178.
872 Recipe No. 23. See the notes on sec. 49.
