506 LECTURE LII. 



[The theory of heat may be said to rest where it did at the time these 

 lectures were written. The facts which have just been mentioned clearly 

 point out its undulatory character. But in what way the major part of 

 the ordinary phenomena al-e to be explained by this doctrine does not yet 

 appear. We have no satisfactory explanation of expansion, of tardy con- 

 duction, of the change of form of certain crystals, of latent heat, and the 

 like. Neither will a merely undulatory hypothesis relieve us from some of 

 the difficulty attendant on certain of the very phenomena which appeared 

 to suggest it ; such as the difference between solar and terrestrial heat, 

 which Melloni shows to depend on the different mixture of the different 

 rays ; the effect of different screens which variously affect different kinds 

 of heat,* the sifting of heat by a screen, so as to render it more capable of 

 transmission through another similar screen. These facts appear to 

 demand a corpuscular theory, wholly or partly accompanied by transverse 

 vibrations. The hypothesis which I have advancedt is, that heat is due to 

 the existence of repulsive atoms which penetrate all material substances ; 

 so that expansion arises from the accumulation of such atoms ; but that 

 the transmission of heat is partly effected by transverse pulses, very nearly 

 in the same way as the tidal water is conveyed up a channel, and accumu- 

 lates at its upper extremity. Solar heat is transmitted altogether by such 

 transverse pulses, so that its intensity is measured by the intensity of the 

 pulses, whilst the heat of a fire is perhaps due in part to normal ones, 

 or, which is the same thing, to a flow of atoms impelling by their repul- 

 sion those wliich are in advance of them. 



The reader will find Professor Forbes's researches in the Edinburgh 

 Transactions, vols. xiii., xiv., and xv. ; and in the Philosophical Maga- 

 zine, vol. vi., &c. Melloni' s, in the Annales de Chimie, vol. liii., &c. ; or in 

 the Scientific Memoirs, vols. i., ii., &c. He may also consult Powell's 

 Reports on Radiant Heat. Reports of the British Association, 1832, 1840.] 



LECT. LII. ADDITIONAL AUTHORITIES. 



Thermometers, fyc. Fludd, De Philos. Moysiaca, 1638. Boyle's Works. Hooke's 

 Statical Therm. Birch, ii. 1. Wallis and Beale, Ph. Tr. 1669, p. 1113. Lahire, 

 Hist, et Mem. 1706, p. 432 ; 1710, p. 546, H. 13 ; 1711, p. 144, H. 40. Amon- 

 tons (air then), ibid. 1703, p. 101, H. 6. Taylor on the Expansion of Fluids, 

 Ph. Tr. 1723, p. 291. Bulfinger, Comm. Petr. iii. 196, 242, iv. 216. Reaumur, 

 Hist, et Mem. 1730, p. 452 ; 1731, p. 250, H. 6. Delisle, Ph. Tr. 1736, p. 221. 

 Ellicott's Pyrometer, ibid. 1736, p. 297. Weitbrecht, Comm. Petr. viii. 310. 

 Krafft, ibid. ix. 241. Ludolff, Mis. Berl. 1740, p. 255. Grischow, ibid. 1740, 

 p. 267. Celsius, Schwed. Abhand. 1742, p. 197. Wargentin, ibid. 1749, p. 167. 

 Smeaton's Pyrometer, Ph. Tr. 1754, p. 598. Lord Cavendish on Max. and Min. 

 Ther. ibid. 1757, p. 300. Bergen, DeTher. 4to, Nuremb. 1757. Sulzer, Act. Helv. 

 iii. 259. Hennert, Traite des Therm. Hague, 1768. Haubold, De Therm. Reaum. 

 4to, Leipz. 1771. Strohmeyer, Ueber die Ther. Gott. 1775. Roy on Ramsden's 

 Pyrometer, Ph. Tr. 1785, p. 461. Report on Ther. ibid. 1777, p. 816. Van 

 Swinden, Comparaison des Ther. Amst. 1778. Schuckburgh, Ph. Tr. 1779. Luz, 

 Ueber die Ther. Nuremb. 1781. Six's Register, Therm. Ph. Tr. Ixxii. 1794, 

 Rutherford's Register, Ed. Tr. iii. 247. Rumford's Differential Ther. Ph. Tr. 1804, 

 p. 77. Dalton, Nich. Jour. 8vo. v. 34. Daniell's Pyrometer, Quarterly Journal of 



* Melloni, Annales de Ch. Iv. 337. 



f Kelland's Theory of Heat, art. 155, 169, 194. 



