ON THE SOURCES AND EFFECTS OF HEAT. 495 



suffering them to cool more gradually, they are rendered softer and more 

 flexible, and the more as the heat which is thus applied is the more consi- 

 derable. [The oxid] which forms itself on the surface of polished iron or 

 steel, serves as a test of the degree of heat which is applied to it, the yellow- 

 ish colour which it assumes indicating the first stage of tempering, the 

 violet the second, and the blue the last ; and if the heat be raised till the 

 surface becomes grey, the steel will be rendered perfectly soft. The density 

 of metals is also a little increased by tempering them, probably for the same 

 reason as water is more dense than ice. In what manner the unequal 

 distribution of the mutual actions of the particles of bodies contributes to 

 increase their hardness, cannot be very positively ascertained, although 

 some conjectures might be formed which would, perhaps, be in some mea- 

 sure explanatory of the facts : but it is safer, in the present state of our 

 knowledge, to be contented with tracing the analogy between these effects 

 in substances of different kinds, and under different circumstances, without 

 attempting to understand completely the immediate operation of the forces 

 which are concerned. 



LECT. LI. ADDITIONAL AUTHORITIES. 



Treatises on Heat.Royle, De Frigore, 4to, Lond. 1683. Petit sur le Froid et 

 le Chaud, 1671. Casatus, De Igne,Leipz. 1688. Muller, De Frigore, 4to, Jena, 

 1698. Winckler, De Frigore, 4to, Leipz. 1737. Chatelet, Diss. sur le Feu, 1744. 

 Bikker, De Igne, 4to, Utr. 1756. Hillary on Fire, Lond. 1760. Belgrado, Del 

 Calore, Parma, 1764. Herbert, De Igne, Vienn. 1773. Marat, Decouvertes sur le 

 Feu, 1779. Recherches sur le Feu, 1780. Magellan sur la NouveUe Theorie du 

 Feu, 4to, Lond. 1780. Scheele, Traite de 1'Air et du Feu (Jr.), 1781. Hopson on 

 Fire, 1781. Baader, Vom Warmestoff, Vienn. 1786. Carradori, Teoriadel Calore, 

 2 vols. Flor. 1787. Berlinghieri, 4to, Pisa, 1787. Marne, Ueber Feuer, 1787. 

 Weber, do. Landshut, 1788. La Serre, Theorie du Feu, Avignon, 1788. Seguin 

 sur les Phenomenes du Calorique. De Luc, Lettres Physiques, 5 vols. Leseme- 

 lier sur 1'Air et le Feu, 2 vols. Paris, 1788. Lorenz Untersuchung des Feuers, 

 Kopen. 1789. Mayer, Ueber die Gesetze des Warmestoffs, Erlang, 1791. Lampa- 

 dius, Ueber Electr. und Warme, Berl. 1793. Voigt, Theorie des Feuers, Jena, 

 1793. Lichtenberg in Erxleben's Naturlehre, 1794 ; Gottling, Weimar, 1794. 

 Harrington on Fire, 1796. Maugin, Theorie du Feu, 1800. Berthollet, Essai de 

 Chimie Statique, 1803. Paulet, Diss. sur le Feu, Lausanne, 1807. P. Prevost, 

 Traite de Calorique Rayonnante, 1809. Oersted, Ansicht der Chemischen Natur- 

 gesetze, Berl. 1812. Pasley's Treatise on Heat, 1820. Paulsen, De Caloris 

 Theoria, Gott. 1821. Fourier, Theorie Analytique de la Chaleur, 4to, 1822. 

 Nobili, Nuovi Trattati sopra il Calorico, &c. Modena, 1822. Bournou, Obs. et 

 Reflex, sur la Calorique, Paris, 1824. Library of Useful Knowledge, Cab. Cyc. &c. 

 Peclet, Traite de la Chaleur et de ses Applications, 2 vols. Paris, 1828. Quetelet, 

 Physique Populaire de la Chaleur, 12mo, Bruxelles, 1832, Bischoff, Warmelehre, 

 1837. Kelland, Theory of Heat, Camb. 1837. Thomson on Heat, 1840. Gehler's 

 Physikalisches Worterbuch, 1841 ; and Gmelin's Handbuch, art. Warme, 1843, are 

 the most complete treatises on the subject. 



Radiation. Newton, Ph. Tr. 1701, p. 827; Opusc. ii. 422. Martine's Essays, 

 1740, p. 236. Rickmann, Nov. Com. Petr. i. 174, 195, U. 172. Lambert, Act. 

 Helv. ii. 172. Erxleben, Nov. Com. Gott. 1777, p. 74, and the authorities given at 

 p. 1636. Rumford, Ph. Tr. 1804, p. 90. Maycock, Nich. Jour. 1810, vol. xxvi. 

 Delaroche, Jour, de Phy. Ixxv. 201. Berard, Ann. de Ch. Ixxxv. 309. Powell, 

 Ph. Tr. 1825, and Report of Br. Ass. vols. i. and ix. 



Conducting Powers. Ingenhousz, Nouvelles Experiences, Par. 1789. Humboldt, 

 Jour, de Phy. xliii. 304. Meyer, Gren's Jour. iv. 22. Biot, Traite de Phy. vol. iv. 

 Despretz, Ann. de Ch. xix. 97. Delarive, ibid. xl. 91. 



