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CHAPTER III.

THF. ALKALIES_THE EARTHS, AND THEIR SALTS.

126. The class of substances called bases, or thosewith which acids can combine, are very numerous.The commonest, and therefore the most important, arethe three alkalies, potash, soda, and ammonia, togetherwith a few substances called earths, because they are theprincipal ingredients which enter into the composition ofthe soil in which plants grow.

127. The principal source of potash is the vegetablekingdom, and the simplest method of procuring it is byburning wood. When this is done, after all the carhon,hydrogen, and oxygen is burned, there remains a quantityof white or grey matter called ashes, which when putinto water renders it caustic. This is occasioned by thepresence of potash, which not being altered by the heatoccasioned by the burning of the wood, is left in theashes.

128. In this way, however, we do not obtain thepotash pure, because, being a powerful base, and havinga strong affinity for acid, it combines with some of thecarbonic acid formed by the burning of the carboncontained in the wood, and therefore the caustic substancefound in the ashes of the wood is an impure carbonateof potash. It is purified by pouring water on ashes,straining off the clear liquor, and evaporating it; a