CH. xxxvn. D ALTON'S ATOMIC THEORY. 403 



whole atom weighing 1 6 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 

 powder, which is caustic soda ; and if you decompose this 

 soda, you will get out of it one measure of hydrogen, one 

 of oxygen, and one of sodium. The sodium, you observe, 

 has turned exactly half the hydrogen out of the water and 

 taken its place ; and this shows there must have been two 

 atoms of hydrogen in the water, because a single atom 

 could not have been divided. 



In the soda we have now got the smallest quantity of 

 each element sodium, oxygen, and hydrogen which will 

 combine with any other. You can turn either of these 

 three out of the soda, but you cannot turn out a part of 

 any one of them. Therefore, a molecule of soda is said to 

 be made of one atom of hydrogen weighing i , one atom of 

 oxygen weighing 16, one atom of sodium weighing 23, and 

 28 



