MODERN SCIENCE. 233 



and magnetic force in high altitudes (the result of a BALLOON 

 ASCENSION to the height of 7,016 metres) ; devised the method 

 of determining the density of vapours ; was one of the first 

 to determine the COEFFICIENT OF THE EXPANSION OF 

 GASES, finding it to be 0*00375 between o and 100 C. 

 He enunciated the two laws: I. All gases have the same co- 

 efficient of expansion as air. II. This coefficient is the same 

 whatever be the pressure supported by the gas. Was as great a 

 chemist as a physicist ; proved that gases combine chemically 

 in simple proportions by volume, and that the volume of the 

 product always bears a simple relation to the volumes of the 

 combining gases. 



1781 1868. Brewster made valuable observations on 

 light ; found the LAWS OF POLARISATION of light through 

 reflection (see also Malus), and that OF DOUBLE REFRACTION,, 

 of biaxial crystals, of optical mineralogy, of double refraction 

 by compression ; invented the kaleidoscope depending upon, 

 the property of inclined mirrors; he also, concurrently 

 with Sir J. Herschel and Talbot, carried on a series of ex- 

 periments on the colours produced by different burning lights. 

 In time it was discovered that each substance gives a bright 

 line in the spectrum of a colour particular to itself, so that a 

 particular colour reveals the presence of a particular substance: 

 a blue flame reveals selenium ; intense yellow, sodium ; 

 violet, potassium ; red, strontium a fact upon which the 

 value of spectrum analysis rests. It is to be remembered 

 that colours depend upon the velocity of undulation, the 

 direction of light-waves, and the difference of wave-lengths. 

 Brewster invented the LENTICULAR SPECTROSCOPE ; found 

 the different proportions which the colours of the solar 

 spectrum present according to the lines which produce them ; 

 counted 2,000 dark lines in the spectrum (Fraunhofer had 

 found 600), but soon after him the existence of 3,000 dark 

 lines was ascertained ; improved the lighthouse system through 

 DIOPTRIC APPARATUS. 



I 7%5 1838. Dulong.in collaboration with PETIT, analysed 

 the LAWS OF COOLING, and showed "the velocity of cooling to 

 increase as the difference of temperature between the surrounding 

 medium and the coeling bodies is greater'' He made numerous 



