PROGRESS IN NINETEENTH CENTURY 41 



It is significant to note that the broad question of thermal expan- 

 sion has yet no adequate equation, though much has been done 

 experimentally for fluids by the magnificent work of Amagat (1869, 

 1873, et seq.). 



Heat Conduction 



The subject of heat conduction from a theoretical point of view 

 was virtually created by the great memoir of Fourier (1822), which 

 shed its first light here, but subsequently illumined almost the whole 

 of physics. The treatment passed successively through the hands of 

 many of the foremost thinkers, notably of Poisson (1835, 1837), 

 Lame (1836, 1839, 1843), Kelvin (1841-44), and others. With the 

 latter (1856) the ingenious method of sources and sinks originated. 

 The character of the conduction is now well known for continuous 

 media, isotropic or not, bounded by the more simple geometrical 

 forms, in particular for the sphere under all reasonable initial and 

 surface conditions. Much attention has been given to the heat con- 

 duction of the earth, following Fourier, by Kelvin (1862, 1878), 

 King (1893), and others. 



Experimentally, Wiedemann and Franz (1853) determined the 

 relative heat conduction of metals and showed that for simple bodies 

 a parallel gradation exists for the cases of heat and of electrical con- 

 ductivity. Noteworthy absolute methods for measuring heat conduc- 

 tion were devised in particular by Forbes (1842), F. Neumann (1862), 

 Angstrom (1861-64), and a lamellar method applying to fluids by 

 H. F. Weber (1880). 



Calorimetry 



Practical calorimetry was virtually completed by the researches 

 of Black in 1763. A rich harvest of experimental results, therefore, 

 has since accrued to the subjects of specific, latent, and chemical 

 heats, due in particularly important cases to the indefatigable Reg- 

 nault (1840, 1845, et seq.). Dulong and Petit (1819) discovered the 

 remarkable fact of the approximate constancy of the atomic heats 

 of the elements. The apparently exceptional cases were interpreted 

 for carbon silicon and boron by H. F. Weber (1875), and for sulphur 

 by Regnault (1840). F. Neumann (1831) extended the law to com- 

 pound bodies, and Joule (1844) showed that in many cases specific 

 heat could be treated as additively related to the component specific 

 heats. 



Among recent apparatus the invention of Bunsen's ice calorimeter 

 (1870) deserves particular mention. 



