The author, after alluding to his previous communications to the Society in reference to fluorine, stated that he had always attributed the slight indications of the presence of this element in plants, which his own investigations and those of others had yielded, to the amount of silica which was contained in vegetable ashes. The presence of silica, which throws special difficulties in the way of detecting fluorine, had also prevented him from seeking for it in trap rocks and other mineral masses. Recently, however, he had put in practice two processes, which were applicable to all bodies containing silica and a metallic fluoride, which are decomposed by boiling oil of vitriol.