376 NINETEENTH CENTURY. FT. in. 



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 oxy- 

 gen weighing 1 6, one atom of sodium weighing 23, and these 

 numbers are called the atomic weights of hydrogen, oxygen, 

 and sodium. 



This will give you a rough idea of Dalton's theory of 

 atoms. There is always this difficulty in it that we cannot 

 be quite sure when we have arrived at the smallest quantity 

 of any substance ; for suppose that one day we were to find 

 that oxygen could be split up into two substances, then it 

 would no longer be true that an atom of oxygen could not 

 be divided. It would then be made up of two elements, the 

 smallest quantities of which, when joined together, would 

 weigh 1 6. But if we bear this possibility in mind, then the 

 theory is of great use in giving us the symbols which are now 

 used in chemical language. For when it was once agreed 

 that the weight of an atom of hydrogen should be reckoned 

 as i, then an atom of oxygen will weigh 16, and the letters 

 HHO express a great deal. They tell us that two atoms of 

 hydrogen weighing 2 are joined to one atom of oxygen weigh- 

 ing 1 6, to fcrm a molecule of water. In the same way 



