THE ELEMENTS. 31 



draught, we have two solutions mixed together, which 

 are able to act on each other, but cannot do so in the 

 dry state ; when water is added they are brought 

 thoroughly in contact, and chemical action at once 

 takes place. In many cases combination is greatly 

 assisted by heat, which exalts the chemical aflSnity that 

 the substances have for each other (35). 



15. Precisely in the same way decomposition is 

 often curiously affected by various circumstances. 

 Some compounds decompose spontaneously ; they 

 cannot be kept any time, and without any external 

 influence they undergo decomposition ; this is espe- 

 cially the case with many animal and vegetable 

 substances (354). Decomposition is frequently caused 

 by the influence of light (187, 295, 699), or heat (9, 

 119, 233). The decomposition of many compounds, 

 likewise, is caused by the presence of particular sub- 

 stances. A great number of different organic sub- 

 stances are decomposed when a small, quantity of 

 some other substance in an active state of decom- 

 position is mixed with them ; this may be called 

 decomposition by example ; it is a very singular form 

 of decomposition, and is termed fermentation (365). 

 A similar change is sometimes caused by the mere 

 presence of a particular substance, even though it is 

 not itself undergoing any change whatever at the 

 time (361). 



16. It is a common saying that there are only four 

 elements ; air, earth, fire, and water ; and many 



