DECOMPOSITION. 29 



to that wliich we have just been describing. It takes 

 place whenever any substance is separated or divided 

 into its component parts. Thus, to return again to 

 the quicklime, which is made by burning chalk or 

 limestone, we say that the chalk or limestone is decom- 

 posed, when, by burning or heating it in a very hot 

 fire, whatever it contains which can be roasted out 

 by fire, is driven off, and the lime only remains 

 (118). 



10. The decomposition of a substance is also effect- 

 ed when it is mixed with anything which has a very 

 strong attraction for one of its components. Soap is 

 made by the attraction which the alkali has for fat 

 (3) ; but if we add to a solution of soap in water, any- 

 thing which has a stronger attraction for the alkali 

 than the latter has for the fat, we shall decompose 

 the soap : there are many substances which have 

 the power of doing this, but it is sufficient now to 

 mention one. If vinegar is poured into a solution of 

 soap, the soap is decomposed ; the fat is separated 

 and floats on the surface, and the vinegar combines 

 with the alkali of the soap. 



11. This kind of change is always going on when 

 anything decays or putrefies, and therefore is of con- 

 siderable interest in connection with manures ; but in 

 fact, combination and decomposition are almost always 

 going on at the same time in most natural changes, 

 for when a compound consisting of several different 

 substances is decomposed, it is generally found that 



3* 



