CH. XXXVI. THE ATOMIC THEORY. 375 



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 there- 

 fore exactly the weight of an atom of any substance will 

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

 on p. 373, 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 

 are supposed to be indivisible, this cannot be done, but a 

 whole atom weighing 16 must be added each time. There- . 

 fore you will see that by an atom Dalton meant the smallest 

 quantity of any element which can combine with other sub- 

 stances. 



Thus, water is made up of molecules, each containing 

 two atoms of hydrogen and one of oxygen. But as these 

 atoms cannot be seen, how can it be known how many there 

 are in any substance, and when we have arrived at the 

 smallest weight of any element ? Dalton knew it in some- 

 thing like the following way : — 



If you decompose water by electricity, you know that you 

 will collect two bottles of hydrogen for one of oxygen. But 

 you can also decompose it another way : if you take a 

 small piece of the metal sodium and float it on water, it will 

 roll round and round fizzing violently. This is because 

 sodium joins very readily to oxygen, and the sodium is 

 turning out some of the hydrogen from the water and taking 

 its place. When the piece of sodium has disappeared, if 

 you evaporate off the rest of the water, you will have a white 



