5 
some Minerals (^ihe ^Zeolite Family. 
another, viz. The variable transparency of different parts of the 
same crystal. You have no doubt observed, that several salts, 
among others the Sulphate of Potash and the Nitrate of Lead, 
yield crystals sometimes transparent and sometimes opaque, and 
of a milky whiteness, without this difference being accompanied 
with any alterations in the proportions of their elements. Among 
the crystals of the Faro apophyllite, there are some which exhi- 
bit milky-white parts, arranged in a regular manner, often in 
the form of a diagonal cross, and easily discernible to the naked 
eye. Differences of this kind, however great be the influence 
which they exercise upon light, can never constitute differences 
of species in mineralogy, — differences which can only be found- 
ed on a real diversity of composition. To distinguish among 
the optical phenomena produced by accidental circumstances, 
and those which are derived from a difference in the elements, 
or in the number of their atoms, is to carry to its maximum the 
employment of optical phenomena as distinctive characters in 
mineralogy. 
Presuming that, in the prosecution of your optical researches, 
you are desirous to know the composition of the other minerals 
which you have had the goodness to send me, I have analysed 
them, and shall now communicate to you the results. 
I began with the examination of the substances which accom- 
pany the Faroe apophyllite. They are inclosed in cavities more 
or less great in the lava which is their gangue. The apophyllite 
is found only in the larger cavities : it and the stilbite appear 
on the anterior surface of the cavities, and have therefore been 
last crystallised. Below and round these two, the interior is co- 
vered with a mamillated mineral, which, in its interior, exhibits 
a radiated and concentric crystallisation. Its colour borders 
a little upon yellow. Below this last, and in immediate contact 
with the lava, is a whiter substance, without any determinate 
marks of crystallisation, though its fracture is grained, and ex- 
hibits irregular facets. This mineral has crystallised first. It 
also fills all the little cavities ; but in the centre of larger cavi- 
ties, we find the radiated mineral, which, in the largest cavities, 
covers entirely the grained mineral, by a stratum considerably 
thicker than that of the latter. These two minerals have a great 
