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ceases immediately after they are combined with bases ;because their powers are then neutralised.
71. A very common illustration of this change ofcolour may he seen in the pickling of red cabbage.Every one knows that red cabbages, as they are called,are really dark purple whilst growing, and they onlybecome red by the action of the acid vinegar employedin pickling them; the same effect would be produced ifany other acid was employed.
72. If now we add to some cabbage thus reddenedby acid a little alkali, either potash, soda, or ammonia,or inde«d a portion of any base, the colour will soon herestored to its original blue ; because the acid is neutral-ised by the base. And if the base employed be one ofthe three alkalies, or their carbonates, such as commonpearlash, which is the carbonate of potash, the solutionof cabbage will become green, because free alkalies, andlikewise their carbonates, have the power of turningvegetable blues green; and thus vegetable colours aremost useful indicators of the presence either of freeacid, or of alkali, in a solution.
73. Although carbonic acid, being a gas, is not per-ceptibly sour to the taste like the strong acids justmentioned, it nevertheless combines with bases, and inso doing neutralises, or at least weakens to a consider-able extent, their caustic properties; but as it has a farless powerful attraction for bases than most otheracids have, it is very easily expelled from compoundscontaining it, by the action of another acid, which