234 PROGRESS OF SCIENCE. 



investigations on elastic forces and elastic fluids ; determined 

 the heat disengaged during combustion by various substances 

 after experiments corroborated by Favre and Silbermann. 



!^86 1853. Arago (D. Francois) showed, with Dulong, 

 that Boyle's law is perfectly exact up to twenty-seven atmos- 

 pheres ; beyond that point of pressure the law suffers an 

 alteration, as Regnault, and also Cailletet, showed. Arago 

 invented, like Rumford, a PHOTOMETER for measuring the 

 intensity of light and investigating its chemical action. He 

 also made numerous observations on chronometers, on 

 thunder, magnetism, electricity, and optical phenomena, and 

 particularly on the action of polarised rays an investigation 

 which he carried on in collaboration with Fresnel and in 

 harmony with Young. As an astronomer, his " MULTIPLE 

 STARS" and ''Astronomical and Geodesical Observations" 

 proved valuable contributions. He measured the arc of the 

 MERIDIAN OF SPAIN. His "Astronomic Populaire" (1834) 

 and his Lectures on Astronomy are perhaps the most 

 beautiful and clearest exposition of this science extant, the 

 magnificent " Outlines of Astronomy " of Sir John Herschel 

 notwithstanding. 



1787 1826. Fraunhofer may be said to have laid the 

 foundations of SOLAR PHYSICS and STELLAR CHEMISTRY ; he 

 was the first (1814), quite independently of Wollaston, to 

 observe carefully, by means of a prism fixed to a telescope, 

 the DARK LINES OF THE SOLAR SPECTRUM (6OO of them). 

 He pointed out their immense significance ; mapped 576 

 LINES, classifying the chief ones (A to G) and showing their 

 relative positions to be constant. He obtained the SPECTRA 

 OF STARS, and pointed out the presence in them of dark lines 

 wanting in our sun's spectrum, and the absence of dark lines 

 present in the solar spectrum inferring thereby a difference 

 of constitution between the sun and the stars. Fraunhofer 

 was therefore the first who studied the celestial bodies by 

 means of spectral analysis. But the dark lines remained a 

 puzzle to scientific men for forty-five years until 1859. (See 

 Bunsen and Kirchhoff.) Fraunhofer, and others after him, 

 found that the spectrum is exhibited under three types : (i) 

 continuous spectrum in which the colours of the prism are 



