. . "Twenty Years' Practical Experience of Natural Asphalt and Mineral Bitumen. [With illustrations.]"@en . . "Twenty years?practical experience of natural asphalt and mineral bitumen"@en . . . . . . . . . . . "Twenty years' practical experience of natural asphalt and mineral bitumen" . "Twenty years Ě“practical experience of natural asphalt and mineral bitumen"@en . . .