xiv THE ATOMIC THEORY 309 



In the sulphate then the ratio of silver to oxygen is 

 100 : 29*650, the corresponding ratio derived from the study 

 of the chlorates is 100 : 44*467. These two quantities of 



oxygen are in the ratio 44 4 7 = 14997. This differs from 

 29*650 



the integral ratio 3 : 2 by only i part in 5000 or 0-02 per 

 cent. 



D. RECIPROCAL PROPORTIONS 



The law of reciprocal proportions. The LAW OF 

 RECIPROCAL PROPORTIONS deals with the case in which two 

 elements combine with a third element, and also with one 

 another. It states that : 



The relative proportions in which two elements combine with 

 a third element are in a simple ratio to those in which they 

 combine with a fourth element or with one another. 



The significance of the law may be illustrated by a simple 

 example. In water, i part of hydrogen is combined with 3 

 parts of oxygen ; in marsh gas it is combined with 3 parts 

 of carbon, whilst in olefiant gas it is combined with 6 parts 

 of carbon, just twice as much as in marsh gas. What will 

 happen if oxygen on the one hand combines with carbon on 

 the other ? The law of reciprocal proportions suggests that 

 these elements will combine with one another in the 

 proportion 



oxygen : carbon 8 : 3 or 8 : 6 



or some simple multiple of these numbers. The two pro- 

 portions suggested by the law are found in fact to be those 

 in which the elements are united in carbonic anhydride and 

 in carbonic oxide respectively ; in a recently-discovered sub- 

 oxide of carbon the proportions are 



oxygen : carbon = 8:9. 

 Again, suppose that oxygen and carbon unite with a fourth 



