"The power of all these substances over vegetable colours is very great in proportion to the quantity used, and it should be remarked, that in this instance also, acids and alkalies, or alkaline earths, are in direct opposition to each other, the acid restoring the colour to its original state, after it has been changed by the alkali, and vice versa. Hence a single coloured vegetable may be made a very delicate test for both acids and alkalies; , for example, previously reddened by acids, will be changed into purple by alkalies, and purpled litmus will detect an acid by becoming red."