"Lastly they all produce certain changes on some vegetable colours, and this test is perhaps the most striking of all above enumerated, since it is possessed by all without exception, though with some variety. The colours of many plants are changed by them to , of which the syrup or tincture of violets, affords a ready instance; many of the reds, such as that of logwood and , are changed into violet; and the of a great many plants, such as , , or root, is changed to a , or dirty brick red."