[753] The heat was measured in Wedgwood's pyrometer by the contraction of pure clay, which is reduced in volume when heated, first by the loss of its water of combination, and afterwards, on the application of more intense heat, by incipient vitrification. The expansion of platina is the test employed by Mr. Daniell in his pyrometer, and this has been found to yield uniform and constant results, such as are in perfect harmony with conclusions drawn from various other independent sources. The instrument for which the author received the Rumford Medal from the Royal Society, in 1833, is described in the Phil. Trans. 1830, part ii., and 1831, part ii.
[754] The above remarks are reprinted verbatim from my third edition, May, 1834. A memoir was afterwards communicated by M. Poisson to the Academy of Sciences, January, 1837, on the solid parts of the globe, containing an epitome of a work entitled "Théorie Mathématique de la Chaleur," published in 1835. In this memoir he controverts the doctrine of the high temperature of a central fluid on similar grounds to those above stated. He imagines, that if the globe ever passed from a liquid to a solid state by radiation of heat, the central nucleus must have begun to cool and consolidate first.
[755] Consolations in Travel, p. 271.
[756] Phil. Trans. 1830, p. 399.
[757] Biblioth. Univers. 1833, Electricité.
[758] Phil. Trans. 1832, p. 176; also pp. 172, 173, &c.
[759] Hist. Mundi, lib. ii. c. 107.
[760] Reduced, by permission, from a figure in plate 40 of Sir H. De la Beche's Geological Sections and Views.
[761] Phil. Trans. 1828, p. 250.
[762] Geology of American Exploring Expedition, p. 369.