ULTIMATE CONSTITUTION OF MATTER 79 



compounds of oxygen, hydrogen, chlorine, lead, etc., are 

 made by the chemist, and then the compounds are isolated 

 in a pure state and analysed, the superfluous substances 

 which have not entered into combination being removed, it 

 will be found that whatever the amount (relatively to any 

 arbitrary standard) of the element utilised, it will always be 

 either this amount, say x or some simple multiple of this 

 amount, say 2^r, 3-r, 43?. For example, nitrogen, which is a 

 monovalent atom, combines with the divalent atom of 

 oxygen to form the compound N 2 O 5 . With more nitrogen 

 and less oxygen, the compounds N 2 O 4 , N 2 O 3 , NO, N 2 O are 

 formed ; but there are no compounds of nitrogen and 

 oxygen which contain a larger proportion of nitrogen than 

 does the compound N 2 O, or a larger proportion of oxygen 

 than N 2 O 5 . So simple are the multiples that if the chemist, 

 wishing to play at making compounds, cuts out blocks of 

 wood and represents each of the elements by a block of a 

 particular colour, he will never need more than six blocks 

 of any given colour to build up all the compounds with 

 which Dalton was acquainted. Since Dalton's time a small 

 number of more complicated combinations, such as the 

 complex silicates and phosphomolybdates, have been dis- 

 covered, but his law still holds good for the vast majority 

 of inorganic substances. This is the basis of the atomic 

 theory. It makes it possible to denote the elements by 

 symbols : O for oxygi n, H for hydrogen, Fe for iron 

 (ferrum) etc., and to express their combinations as OH 2 , 

 FeO, Fe 2 O 3 , Fe 3 O 4 , Fe 2 O 3 H 2 O, etc. 



If any arbitrary unit be chosen, if, for example, it be 

 assumed that the weight of the atom of the lightest element, 

 hydrogen, is i, then it follows that in any compound in 

 which there are exactly the same number of atoms of 

 hydrogen as of some other element, the actual weight of 

 the other element found in the compound is to the actual 

 weight of the hydrogen as the weight of the atom of the 

 other element to the weight of the atom of hydrogen. For 



