30 ^ DECOMPOSITION. 



these substances again combine together, one with 

 another, to form various other compounds (105, 145). 



12. Combination takes place whenever substances 

 are brought together which have an affinity for each 

 other, under suitable circumstances ; chemical action 

 then takes place, and a compound is formed. De- 

 composition of a compound is caused either by the 

 influence of some external power, the presence of 

 some substance capable of acting on one of the ele- 

 ments of the compound, or some influence able to 

 weaken the chemical affinity which binds these ele- 

 ments together. - 



13. Combination is often modified and controlled 

 in a very remarkable manner by circumstances, ac- 

 cordingly as they are favorable or unfavorable to 

 the union of the substances brought together. Fine 

 division, or any other method whereby the particles 

 are enabled readily to mix or come in contact, gener- 

 ally assist combination. When two solid lumps are 

 placed in contact with each other, they are only able 

 to touch by a very few points, and hence in many 

 cases do not combine even though they have an 

 affinity for ea<3h other. If they are finely powdered 

 and well mixed they are much more able to combine ; 

 and for the same reason if they are fusible, heat, by 

 rendering them fluid, and thus enabling the particles 

 more easily to mix, assists their combination, 



14. A similar efi*ect is produced by solution in 

 water. In a common Seidlitz powder or saline 



