Septemder 22, 1905.] 



SCIENCE, 



365 



mounted. The subject is still under vigor- 

 ous discussion, as the papers of Burbury 

 (1899) and others testify. 



To Maxwell (1860, 1868) is due the spe- 

 cifically kinetic interpretation of viscosity,' 

 of diffusion, of heat conduction, subjects 

 which also engaged further attention from 

 Boltzmann (1872-87). Rigorous data for 

 molecular velocity and mean free path have 

 thus become available, and van der Waals 

 (1873) added a final allowance for the size 

 of the molecules. 



Less satisfactory has been the explora- 

 tion of the character of molecular force 

 for which Maxwell, Boltzmann (1872, et 

 seq.), Sutherland (1886, 1893) and others 

 have put forward tentative investigations. 



The intrinsic , equation of fluids discov- 

 ered and treated in the great paper of van 

 der Waals (1873), though partaking of 

 the character 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 inferred by van der Waals from 

 very simple premises. Many of the results 

 have been tested by Amagat (1896). 



The data for molecular diameter fur- 

 nished 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 approxima- 

 tion to the truth has surely been reached. 



RADIATION. 



Our knowledge of the radiation of heat, 

 diathermacy, thermocrosis, was promoted 

 by the perfection which the thermopyle 

 reached in the hands of Melloni (1835-53). 

 These and other researches set at rest for- 

 ever all questions relating to the identity 



of heat and light. The subject was, how- 

 ever, 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 labori- 

 ous 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), per- 

 vades 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 Reichsan- 

 stalt 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 con- 

 stant of solar radiation at the earth was 

 made by Pouillet (1838) ; but other pyr- 

 heliometric methods have since been de- 

 vised by Langley (1884) and more recently 

 by Angstrom (1886, et seq.). 



VELOCITY OF LIGHT. 



Data for the velocity of light, verified 

 by independent astronomical observations, 

 were well known prior to the century-; for 

 Romer had worked as long ago as 1675, 

 and Bradley in 1727. It remained to ac- 

 tually measure this enormous velocity in 

 the laboratory, apparently an extraordi- 

 nary feat, but accomplished simultaneously 

 by Fizeau (1849) and by the aid of Wheat- 

 stone's revolving mirror (1834) by Fou- 



