40 ‘ E. J. CHAPMAN ON THE COLOURING 
The existence of a black allotropic condition of silica (or of any of the other compo- 
nents of these dark tourmalines) is a very gratuitous supposition; especially when we 
consider that where a silicate, as amphibole, pyroxene, epidote, garnet, etc., presents 
several varieties or subspecies differing in intensity of colour, the dark kinds always 
contain a certain amount of iron, whilst the lighter varieties are often entirely free from 
ferruginous matter. 
All things considered, therefore, there can be little doubt that the black colour and 
opacity of schorl is due to the presence of iron in some state of oxidation. The question 
is, in what state? Rammelsberg’s analyses might be thought a sufficient answer; but 
with all due recognition of the high place deservedly occupied by this chemist as a 
mineral investigator, there are certain facts which are altogether opposed to the accept- 
ance of his view that all the iron in black tourmaline is in the condition of monoxide. 
Neither Fe O nor Fe? O°, when completely dissolved in a vitreous flux, produces blackness 
and opacity. As first clearly shewn by Berzelius, Fe’ O* in a borax glass—and the reaction 
is the same in a fusible siliceous glass—imparts to this a yellowish tint. On partial 
reduction, when the Fe 0° becomes Fe’ O', the glass becomes bottle-green ; and with excess 
of oxide it turns black and opaque. But on complete reduction to Fe O (not easily effected 
when much colouring matter is present) the blackness and opacity give place to a clear 
green colour, and the glass becomes more or less transparent. In connection with these 
reactions, one may call to mind the oxidizing effect, and its results, of nitric acid on a 
crystal of ferrous sulphate: the blackening of the crystal at first, and the subsequent 
decoloration on the oxidation becoming complete. In one case the deep color results from 
the reduction of Fe? O° into Fe’ O'; and in the other, from the oxidation of Fe O into the 
latter compound. In both cases the blackness or depth of colour is produced not by Fe O, 
nor by Fe’ O’, but by Fe° O'. 
From these considerations, therefore, supported by the experiments described below, 
I think it may be fairly inferred that the iron in the black and deeply-coloured tourma- 
lines is wholly or essentially present as ferro-ferric oxide. 
It might be thought that if the black tourmalines contain Fe’ O' they should be 
attractable in small splinters by an ordinary magnet, whereas they do not show this 
character; but when a borax-glass, or a boro-siliceous glass, is rendered black before the 
blow-pipe, by excess of ferro-ferric oxide, it does not, even in splinters, show any magnetic 
attraction. ; 
The following experiments (the products resulting from which I have the honour to 
lay before the Society) go far to support the view advocated in this paper, namely, that 
the iron in black tourmaline, from which the colour of the mineral is derived, is essen- 
tially in the condition of ferro-ferric, not of ferrous, oxide. 
(1) A portion of a large, intensely-black crystal of tourmaline was subjected, in an 
open crucible, to a strong heat for a couple of hours. It melted, and became slightly 
brown on the surface. When broken, it showed internally a vesicular structure, and 
dull black colour. Portions of other black crystals treated in this manner gave similar 
results. A slight peroxidation of the surface only was effected. The dark colour prac- 
tically remained unchanged. 
(2) Another portion of the large crystal, used in experiment No. 1 was imbedded in a 
thick mass of filings of black pig-iron in an ordinary clay crucible. Powdered anthracite 
