204 FAMOUS GEOLOGISTS/Hutton forces were such that the ‘schistus’ now stood almost vertically But the strata were then subjected to weathering and erosion so that the upheaved rocks were reduced to an approximately level surface In time, the now vertical strata subsided below the sea once more (by an inadequately explained process) and were covered by layers of sediment derived from adjacent high ground Again there was consolidation, following which uplift occurred, exposing the strata to the elements once more, but without the overlying sandstones being folded or inclined Thus the disposition of the rocks observed at Siccar Point could be understood – provided that time was unlimited Playfair wrote in his biographical memoir of Hutton: Revolutions still more remote appeared in the distance of this extraordinary perspective The mind seemed to grow giddy by looking so far into the abyss of time; and while we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination can venture to follow Playfair (1805, p 73) Thus at Siccar Point Hutton provided evidence for (but not formal proof of) the Earth’s great age and the cyclic nature of geological processes The locality has long been recognized as one of geology’s most significant field sites Hutton acquired Playfair as a convert to his theory, and it was Playfair who popularized Hutton’s ideas – Hutton’s prolix style and confusing theory of heat (see below) did not gain him many adherents Hutton’s Later Work on the Theory of Heat In his old age, Hutton tried to give some kind of physicochemical explanation of the forces causing elevation and subsidence, but he had little success He knew that bodies expanded when heated, and the kind of heat that produced this effect he called ‘sensible heat’ He also knew that when heat was applied to a solid it increased in temperature, but on reaching the melting point it would melt without changing temperature, even though it was still being supplied with heat In the change of state, the heat supplied to produce melting was somehow hidden Black had called this ‘latent heat’ But the nature of heat was uncertain Hutton thought it was a kind of weightless ‘substance’ He knew that everyday objects have mass and that massive bodies are attracted to one another by gravitation But there also seemed to be repulsive forces at work, as for example when water is boiled: steam engines exert pressure in their cylinders Today, we distinguish between radiant heat and heat transmitted by conduction Hutton had no adequate concept of radiation, but he knew that heat from the sun shines on us, across space He called it ‘solar substance’, and, though weightless, it somehow seemed to be absorbed by plants, though Hutton did not know how Adding to the complications, Hutton accepted the old ‘phlogiston theory’ of combustion (which was collapsing at the end of the eighteenth century), according to which an inflammable material contains a weightless ‘substance’ or ‘principle’ called ‘phlogiston’, which is dispersed into the atmosphere during combustion Hutton was inclined to suppose that ‘solar substance’ and ‘phlogiston’ were one and the same (Actually, if one regards ‘phlogiston’ as energy, then some of the problems that Hutton was trying to understand fall into place for us.) Hutton grappled with such problems in two books: Dissertations on Different Subjects in Natural Philosophy (1792); and A Dissertation upon the Philosophy of Light, Heat, and Fire (1794) All his arguments cannot be followed here, but he tried out the idea that objects normally attracted one another according to the inverse-square law of gravitation Thus he spoke of ‘gravitating matter’ At very close quarters, however, objects supposedly began to repel one another, according to a force law in which the distance between particles was raised to a power greater than two The repulsive force (or ‘solar substance’) could supposedly take various guises: ‘sensible’ heat, manifested by expansion; latent heat; light; electricity; and phlogiston So, when sediments were under extreme pressure, they might move from a compressive phase to an expansive (expanding) phase Hence, in the geostrophic cycle, there could be alternating periods of contraction (compression or consolidation) and expansion (producing land elevation) Hutton’s theory depended on a balance of attractive (gravitational, cohesive, and concretive) and repulsive (specific, or sensible, and latent heats) forces There could be different resultant states, arising from the forces producing elevation (expansion) and subsidence (contraction) at different times and places But when, lacking the concept of energy, Hutton started talking about ‘solar substance’ in reference to solar radiation (as we would say) and thought that this ‘substance’ was immaterial, confusion and misunderstanding followed amongst his contemporaries; it is scarcely possible for us to make sense of his theory of heat