Note 30 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

It is necessary to remark that, although arsenic is so closely analogous to phosphorus (especially in the higher forms of combination, RX3 and RX5), at the same time it exhibits a certain resemblance and even isomorphism with the corresponding compounds of sulphur (especially the metallic compounds of the type MAs, corresponding with MS). Thus compounds containing metals, arsenic, and sulphur are very frequently met with in nature. Sometimes the relative amounts of arsenic and sulphur vary, so that an isomorphous substitution between the arsenides and sulphides must be recognised. Besides FeS2 (ordinary pyrites), and FeAs2, iron forms an arsenical pyrites containing both sulphur and arsenic, which from its composition, FeAsS or FeS2FeAs2, resembles the two preceding.