John Dalton 15 



sharp distinction between elementary and compound bodies 

 could be drawn, and even explained the difference between 

 mixtures and chemical compounds. But it was only when 

 phlogiston had been finally abandoned that the way was 

 prepared for our present conception of the constitution of 

 matter. This is indelibly connected with the name of John 

 Dalton (1766-1844), who taught us that the material uni- 

 verse contains a certain number of elementary substances, 

 each possessing, as its ultimate constituent, a distinctive 

 atom which cannot be split up farther by chemical or 

 physical means. There are, therefore, as many different 

 kinds of atoms as there are elementary substances. The 

 atoms of each element are alike in every respect, and have 

 the same weight. When atoms of different elements enter 

 into close union with each other, they form what Dalton 

 called " compound atoms," or, according to our present 

 nomenclature, " molecules " ; these are the ultimate con- 

 stituents of compound bodies. 



Dalton's first scientific interests, which he preserved 

 through life, were connected with meteorology. He was 

 led to his chemical investigations through attempting to 

 find a reason for the uniformity in the mixture of gases 

 at different levels of the atmosphere, being much puzzled 

 to know why the oxygen, nitrogen, and aqueous vapour 

 did not arrange themselves in layers according to their 

 density, as when oil rises to the top if mixed with water. 

 His difficulty was mainly due to the peculiar ideas he had 

 formed of the nature of a gas. For a time he seems to 

 have adopted the correct view that all gases at the same 

 temperature and pressure have the same number of ultimate 

 particles in unit volume, but he abandoned it because it 

 did not seem to him to lead to the observed intermingling 

 of gases irrespective of their density. He then invented 

 a rather fanciful hypothesis which drew a distinction between 

 the density of an atom and its weight, and he tried to 

 find some connexion between the two. This led him 

 to investigate atomic weights. Dalton's temperament and 

 methods of procedure were different from those of the 

 other leaders of science whose work is under review. He 

 is rightly considered to be the originator of the principle 



