The crystals referred to were found in considerable quantity in the slags produced in Mr. Hollwaf's experiments on the reduction of metallic sulphides at Penistone, in July 1878.

The experiments were carried out with ordinary Bessemer plant. Upon running the molten mass from the ladle into one of the ingot moulds the slag ran in a thick stream of perfect continuity, falling without noise a distance of some feet into the liquid portion below. The crystals occurred lining cavities in the partially cooled slag from which a portion of the molten material had been withdrawn.