In 1795 Dr. Hutton published a more elaborate statement of his theory in two volumes. In 1802 Playfair printed _Illustrations of the Huttonian Theory_, a simplification, having, naturally, little originality. Before his death in 1797 Hutton devoted his time to reading new volumes by Saussure on the Alps, and to preparing a book on _The Elements of Agriculture_.
Sir James Hall of Dunglass was a reluctant convert to Hutton's system of geology. Three arguments against the Huttonian hypothesis gave him cause for doubt. Would not matter solidifying after fusion form a glass, a vitreous, rather than a crystalline product? Why do basalts, whinstones, and other supposedly volcanic rocks differ so much in structure from lava? How can marble and other limestones have been _fused_, seeing that they are readily calcined by heat? Hutton thought that the compression under which the subterranean heat had been applied was a factor in the solution of these problems. He was encouraged in this view by Black, who, as already implied, had made a special study of limestone and had demonstrated that lime acquires its causticity through the expulsion of carbonic acid.
Hall conjectured in addition that the rate at which the fused mass cooled might have some bearing on the structure of igneous rocks. An accident in the Leith glass works strengthened the probability of his conjecture and encouraged him to experiment. A pot of green bottle-glass had been allowed to cool slowly with the result that it had a stony, rather than a vitreous structure. Hall experimenting with glass could secure either structure at will by cooling rapidly or slowly, and that with the same specimen.
He later enclosed some fragments of whinstone in a black-lead crucible and subjected it to intense heat in the reverberating furnace of an iron foundry. (He was in consultation with Mr. Wedgwood on the scale of heat, and with Dr. Hope and Dr. Kennedy, chemists.) After boiling, and then cooling rapidly, the contents of the crucible proved a black glass. Hall repeated the experiment, and cooled more slowly. The result was an intermediate substance, neither glass nor whinstone--a sort of slag. Again he heated the crucible in the furnace, and removed quickly to an open fire, which was maintained some hours and then permitted to die out. The result in this case was a perfect whinstone. Similar results were obtained with regular basalts and different specimens of igneous rock.