PROGRESS IN NINETEENTH CENTURY 45 



ter of a first approximation, has greatly promoted the coordination 

 of most of the known facts. Corresponding states, the thermal 

 coefficients, the vapor pressure relation, the minimum of pressure- 

 volume products, and even molecular diameters, are reasonably in- 

 ferred by van der Waals from very simple premises. Many of the 

 results have been tested by Amagat (1896). 



The data for molecular diameter furnished by the kinetic theory 

 as a whole, viz., the original values of Loschmidt (1865), of van der 

 Waals (1873), and others, are of the same order of values as Kelvin's 

 estimates (1883) from capillarity and contact electricity. Many 

 converging lines of evidence show that an approximation to the 

 truth has surely been reached. 



Radiation 



Our knowledge of the radiation of heat, diathermacy, thermo- 

 crosis, was promoted by the perfection which the thermopyle reached 

 in the hands of Melloni (1835-53). These and other researches set 

 at rest forever all questions relating to the identity of heat and light. 

 The subject was, however, destined to attain a much higher order 

 of precision with the invention of Langley's bolometer (1881). The 

 survey of heat spectra, beginning with the laborious attempts of 

 Herschel (1840), of E. Becquerel (1843, 1870), H. Becquerel (1883), 

 and others, has thus culminated in the magnificent development 

 shown in Langley's charts (1883, 1884, et seq.). 



Kirchhoff's law (1860), to some extent anticipated by Stewart 

 (1857, 1858), pervades the whole subject. The radiation of the black 

 body, tentatively formulated in relation to temperature by Stefan 

 (1879) and more rigorously by Boltzmann (1884), has furnished 

 the savants of the Reichsanstalt with means for the development 

 of a new pyrometry whose upper limit is not in sight. 



Among curious inventions Crooke's radiometer (1874) and Bell's 

 photophone may be cited. The adaptation of the former in case of 

 high exhaustion to the actual measurement of Maxwell's (1873) 

 light pressure by Lebedew (1901) and Nichols and Hull (1903) is 

 of quite recent history. 



The first estimate of the important constant of solar radiation at 

 the earth was made by Pouillet (1838); but other pyrheliometric 

 methods have since been devised by Langley (1884) and more re- 



o 



cently by Angstrom (1886, et seq.). 



Velocity of light 



Data for the velocity of light, verified by independent astronom- 

 ical observations, were well known prior to the century; for Romer 



