162 THE HISTORY OF SCIENCE 



yield to a method introduced by the great Swedish 

 chemist Berzelius. In 1837 Dalton wrote : " Ber- 

 zelius's symbols are horrifying : a young student in 

 chemistry might as soon learn Hebrew as make him- 

 self acquainted with them. They appear like a chaos 

 of atoms . . . and to equally perplex the adepts of 

 science, to discourage the learner, as well as to cloud 

 the beauty and simplicity of the Atomic Theory." 



Meantime Dalton's mind had been turning to the 

 consideration of the relative sizes and weights of the 

 various elements entering into combination with one 

 another. He argued that if there be not exactly the 

 same number of atoms of oxygen in a given volume 

 of air as of nitrogen in the same volume, then the 

 sizes of the particles of oxygen must be different 

 from those of nitrogen. His interest in the absorp- 

 tion of gases by water, in the reciprocal diffusion of 

 gases, as well as in the phenomena of chemical com- 

 bination, stimulated Dalton to determine the relative 

 size and weight of the atoms of the various elements. 

 Dalton said nothing of the absolute weight of the 

 atom. But on the assumption that when only one 

 compound of two elements is known to exist, the 

 molecule of the compound consists of one atom of 

 each of these elements, he proceeded to investigate 

 the relative weights of equal numbers of the two 

 sorts of atoms. In 1803 he pursued this investiga- 

 tion with remarkable success, and taking hydrogen 

 (the lightest gas known to him) as unity, he arrived 

 at a statement of the relative atomic weights of 

 oxygen, nitrogen, carbon, etc. Dalton thus intro- 

 duced into the study of chemical combination a very 

 definite idea of quantitative relationship. By him 



