FBASIUS.                                   265
	The ancient Prasius, so called from the resemblance of its colour to the peculiar green of the leek (ðñÜóéïí) is now confounded in the numerous class of green stones styled indiscriminately Plasma by the antiquary. This word, formerly written Prasma, whence the French name, Prisme d'Émeraude (the origin of the common, but deceptive designation Boot of Emerald) is merely the Italian cor­ruption of Prasina, according to their common vulgarism of changing R into L and vice versa. Under the Lower Empire, when the slang of the circus had become the current language, Prasinus, the colour of the Green faction, came to denote that particular shade to the exclusion of the more general word Viridis. Hence we find the pane­gyrist (in Symmachus) using the word Prasini instead of Smaragdi ; and Epiphanius (' XII. Stones of the Breast­plate,' iii.) remarking, by a slight blunder, that the Smaragdus is called also the Prasius. The Plasma, considered mineralogically, is merely trans­lucent Calcedony coloured green by some metallic oxide, of copper when dark, of nickel when of a brighter shade. It is in fact a nearly transparent green Jasper, and though frequently approximating in colour to the finest Emerald, it is seldom quite pure, but marked with black spots, or yellow patches, betraying its proper species. In fact, some epecimens are internally diversified with ramifications of a dark, opaque, green, so as to deserve the name of green-