The Bakerian lecture.—Researches on venadium.—Part I

Abstract

Amongst the physical properties which point out the general relationship and classification of chemical compounds, none has so deservedly obtained the confidence of chemists as isomorphism. The vanadium compounds have, however, proved a remarkable and unexplained exception to the conclusions which generally follow from well ascertained identity of crystalline form. Rammelsberg, and afterwards, more completely, Schabus, pointed out the fact that the mineral vanadinite from several localities (a compound of lead vanadate and lead chloride) is isomorphous with apatite, pyromorphite, and mimetesite, minerals consisting of calcium phosphatochloride, lead phosphato-chloride, and lead arsenato-chloride. The crystalline form of all these minerals is an hexagonal prism, terminated by 6-sided pyramids. So far indeed has the isomorphism of these compounds been traced, that in many specimens these minerals have been found to crystallize together in all proportions; and Heddle describes a crystal in his possession, the upper half of which consists of vanadinite and the lower half of pyromorphite.