ESSAY XV. 



OF THE DECOMPOSITION OF NEUTRAL SALTS BY 

 UNSLAKED LIME AND IRON. 1779. 



IT is looked upon as a demonstrated truth in chemistry, 

 that fixed alkalies have a stronger attraction for acids than 

 absorbent or metallic earths, and that the latter are precipit- 

 ated by the former. There seem to be only two exceptions 

 to this rule, namely, in favour of ponderous earth, which 

 has a stronger affinity for all acids, and lime, which unites 

 more readily with most acids than alkalies do. That the 

 foregoing tenet should be still further limited, I have dis- 

 covered by the following experiments. 



I once found in a cellar a wooden vessel hooped with 

 iron hoops, and containing salted turnips. The iron hoops 

 were covered over with a salt which appeared perfectly to 

 resemble mineral alkali. This accident appeared very 

 singular, as I well know that the acid of salt has a weaker 

 attraction for iron than for mineral alkali ; I could not 

 therefore believe that the common salt, oozing out through 

 the wood, could be decomposed by the iron hoops. To 

 resolve this doubt, a clean plate of iron was dipped into a 

 saturated solution of common salt, and hung up in a moist 

 cellar. In fourteen days mineral alkali was found on the 

 plate. There appeared also some yellow drops containing 

 iron, which was precipitated when a little of the alkali that 

 was beside them was brought close to them. I afterwards 

 covered over another iron plate with a saturated solution 



