D ALTON 119 



of the relative weights of atoms. The numbers he ob- 

 tained as representing the atomic weights were in many 

 cases erroneous ; but they were rectified by the work of 

 Berzelius (1779-1848), and even now these constants of 

 nature are subject to frequent revision. It was, however, 

 to the genius of Dal ton that the atomic weights of the 

 elements were first comprehended. 



Since Dalton's time, the sizes, intervals, and velocities 

 of atoms have been ascertained. These problems have 

 been solved by Clausius, Kelvin, Clerk Maxwell, and 

 others from various sides: "from a comparison with the 

 wave-lengths of light, with the tenuity of the thinnest 

 films of soap-bubbles just before they burst, and from 

 the kinetic theory of gases, involving the dimensions, 

 paths, and velocities of elastic bodies, constantly col- 

 liding, and by their impacts producing the resulting 

 pressure on the confining surface." For instance, one 

 cubic centimetre of air contains twenty-one trillions of 

 molecules ; the average distance between each molecule 

 equals ninety-five million ths of a millimetre ; the average 

 velocity of each molecule is four hundred and forty-seven 

 metres per second ; and the average number of impacts 

 received by each molecule is four thousand seven hundred 

 millions per second. 



In 1865 Loschmidt of Vienna, twenty-one years after 

 Dalton's death, calculated that the diameter of an atom 

 of oxygen was the one-ten-millionth of a centimetre ; 



