402 NINETEENTH CENTURY. PT. in. 



all the elements, each one having its own peculiar weight. 

 Nitrogen, for example, combines in weights of 14, or twice 

 14 = 28, or three times 14 = 42, etc. ; sodium in weights of 

 23, 46. and 69, etc. This is called the law of multiple pro- 

 portions, which we owe entirely to Dalton, and it is a fact 

 about which all chemists agree. Dalton went on to try and 

 explain it by a theory which is still a matter of speculation, 

 and which some chemists do not receive. 



Dalton's Atomic Theory, 1808. In order to explain 

 why each element should have its fixed weight in which it 

 always combines, Dalton imagined, as Democritus, Epicurus, 

 IJacon, and Newton had done before him, that all matter is 

 composed of tiny parts, or atoms, which are too small to be 

 seen, and which cannot be divided. These atoms, which he 

 pictured to himself as round grains like very small shot, 

 would be of the same size in every substance, but not of the 

 same weight. Hydrogen atoms would be the lightest of 

 all, for hydrogen is the lightest substance known ; oxygen 

 atoms would be 16 times, and nitrogen 14 times, as heavy 

 as those of hydrogen. 



Now when two elements combine together they cannot 

 take up less, according to Dalton, than one atom of each, 

 or two atoms of one to one of the other, and so on, and 

 therefore exactly the weight of an atom of any substance will 

 always be added. For example, to turn back to our table 

 on p. 389, Dalton would say that a molecule, or the smallest 

 portion which can be imagined, of nitrous oxide will contain 

 2 atoms of nitrogen weighing 14 each to i atom of oxygen 

 weighing 16 ; while nitric acid will contain 2 atoms of nitro- 

 gen = 28, and 5 atoms of oxygen, 5 x 16 = 80. If half an 

 atom of oxygen could be added, then it might be possible 

 to take up 16 + 8, or 24 parts of oxygen ; but as the atoms 

 arc. supposed to be indivisible, this cannot be done, but a 



