THE PATH OF EVOLUTION 



two elements, as when sulphur and iron when heated 

 form iron sulphide. 



3d. Decomposition by substitution. When to a 

 substance composed of two or more different atoms 

 or molecules the molecules of a third substance are 

 brought, whose affinities for one of the constituent 

 molecules of the first substance is stronger than that ex- 

 isting between the molecules already in combination, 

 then the third element will displace one of the former, 

 setting it free. Thus zinc, when added to hydro- 

 chloric acid (a compound of chlorine and hydrogen), 

 will unite with the chlorine, forming Zinc Chloride, 

 displacing the hydrogen, which escapes in the free 

 state. 



4th. Double decomposition. When to a substance 

 composed of the union of two different molecules is 

 added another substance whose two component mo- 

 lecules have stronger affinities for those that compose 

 the former body, there will be formed new bodies 

 from the mutual interchange of the molecules respec- 

 tively of the two original substances. For instance, 

 when a solution of silver nitrate is brought into con- 

 tact with a solution of sodium chloride, the result is 

 the formation of sodium nitrate and the precipitation 

 of the insoluble Silver Chloride. 



As already stated, the changes of chemical affinity 

 are accompanied or caused by the manifestation of 



