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strong heat for its fusion, and possessed of a highly 
caustic taste. In the matter usually called pure 
potassa by chemists, it exists, combined with water : 
and in that commonly called pearl-ashes, or pot- 
ashes in commerce, it is combined with a small 
quantity of carbonic acid. Potassa in its uncom- 
bined state, as has been mentioned, page 48. con- 
sists of the highly inflammable metal potassium and 
oxygene, one proportion of each. 
Soda, or the mineral alkali, is found in some 
plants that grow near the sea ; and is obtained 
combined with water, or carbonic acid in the 
same manner as potassa ; and consists, as has been 
stated, page 48. of one proportion of sodium, 
and two proportions of oxygene. In its properties 
it is very similar to potassa ; but it may be easily 
distinguished from it by this character ; it forms a 
hard soap with oil : potassa forms a soft soap. 
Pearl ashes, and barilla and kelp, or the impure 
soda obtained from the ashes of marine plants, are 
very valuable in commerce, principally on account 
of their uses in the manufacture of glass and soap. 
Glass is made from fixed alkali, flint, and certain 
metallic substances. 
To know whether a vegetable yields alkali, it 
should be burnt, and the ashes washed with a small 
quantity of water. If the water, after being for 
some time exposed to the air, reddens paper tinged 
with turmeric, or renders vegetable blues green, 
it contains alkali. 
To ascertain the relative quantities of pot-ashes 
afforded by different plants, equal weights of them 
should be burnt : the ashes washed in twice their 
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