116 BIOGRAPHIES OF SCIENTIFIC MEN 



ducting power of water for heat ; and in 1801 appeared his 

 Constitution of Mixed Gases, wherein he proved " the 

 total pressure of a mixture of two gases on the walls of 

 the containing vessel is equal to the sum of the pressures 

 of each gas ; in other words, that if one gas is removed 

 the pressure now exerted by the remaining gas is exactly 

 the same as was exerted by that gas in the original 

 mixture." 



It may be mentioned that among Dalton's pupils was 

 the celebrated James Prescott Joule (of the " mechanical 

 equivalent of heat " and the " conservation of energy " 

 fame). Both tutor and pupil are in the first rank of 

 scientific investigators, and hence Manchester has the 

 perhaps unique distinction of having been the home of 

 two of the greatest natural philosophers who ever lived. 



John Dalton, the Quaker philosopher, was the founder 

 of the atomic theory, and this great generalization is one 

 of the foundation-stones of modern chemistry. 



Leucippus appears to have been the first to grasp the 

 idea that matter is composed of ultimate particles or 

 atoms. Democritus of Abdera (born 460 B.C.) developed 

 the atomic theory of Leucippus, and stated that atoms 

 were impenetrable and indivisible. Epicurus (born 

 340 B.C.) gave mobility to the atoms, and otherwise 

 greatly improved the atomic theory of the Greek 

 philosophers. According to these sages " the world is 

 composed of an innumerable quantity of atoms, mobile, 



