116 ACTION OF SALTS. 



of alumina, the least of all demanded oy plants. Long 

 formed and sun-baked, they are scarcely acted on by rain or 

 dew, and are almost useless. Here, lime, by decomposing 

 these earthy and metallic geates, forms a combination 

 which, in its nascent state, is readily dissolved. If the car- 

 bonate of lime acts better than the hydrate, it is because, 

 following a well-known law, double decomposition is easier 

 than single. If any acid geine exists in the soil, or any free 

 acids, carbonic acid is then liberated, it acts on the geate of 

 lime, super-geates result, and these are easily soluble. 



3d. The great use of lime is as a converter, turning solid 

 and insoluble geine, even solid vegetable fibre, into soluble 

 vegetable food. Here is the point, where philosophy seems 

 to give the choice, to refer this action to one of the numer- 

 ous cas'es of catalytic change, which are every day becoming 

 more and more familiar ; or to explain the whole process by 

 referring it to saponification. This word is used as convey- 

 ing at once what is meant, but it is not meant to say that 

 the product of lime and vegetable matter is soap. The 

 action of lime on geine may be similar to its action on oil 

 and fat. It is well-established that animal and vegetable oils 

 and fiits are converted into acids by the action of alkalies, 

 earths, oxides, and even by vegetable fibre itself The gen- 

 eral law is, that whenever a substance capable of uniting 

 with the acid of fat or oil, is placed in contact with fat or 

 oil, it determines the production of acid. Now, it has been 

 shown that alkali produces a similar change on geine, it de- 

 velops acid properties. If alkali has converted vegetable oil 

 and geine into acid, it is a reason why a similar action may 

 be produced by all those substances which act thus on oil. 

 Hence lime, earths, and metallic oxides, convert geine .into 

 acid ; as fast as this takes place, so fast it becomes soluble. 

 Then, too, the long action of air on insoluble geine render- 



