M. Beudant on the Opals of Hungary. S3T 
tioned, since, from the manner in which it occurs inclosed in the 
rocks, it could not have been exposed to the influence of the air. 
I am rather of opinion that it is a particvdar state of opal. The 
workmen also distinguish these earthy parts, which they regard 
as opal that is not yet ripe, from those which are produced by 
the exposure of opal to the air, which they name burnt or calci- 
ned opal. It is to be remarked, in support of the distinction 
which I have deemed it proper to establish, that these matters 
harden a little on exposure to the air, and crack in collections^ 
precisely in the same way as alumine or silica, in a state of 
jelly, which are desiccated in our laboratories. The same cir- 
cumstance takes place with regard to substances, which ap- 
proach in character to the opals already described. This I 
have observed, on the one hand, near Ribnik, in the fissures of 
semivitreous trachyte ; on the other, at Kremnitz, in the ca- 
vities of a cellular trachyte, inclosed in conglomerates. It has 
been, without doubt, observations of this kind which have led 
certain authors to say, that opals are found, when in the 
bowels of the earth, so soft as to receive the impression of 
the fingers, and that they harden only by exposure to the 
jur, This idea is not, perhaps, so ridiculous as might at first 
be imagined; for we know that silica in solution, assumes, 
on drying, a certain degree of hardness, and a lustre ap- 
proaching to that of opal. It is true, that the greater num- 
ber of opals are solid, when taken from the rock ; but, after 
finding them occasionally still soft, and capable of drying in the 
air, might it not be supposed that the rest have undergone this 
desiccation in a slower manner in the bowels of the earth ? By 
admitting this hypothesis, we can discover the reason of the 
diflerence which exists between the hyaline quartz and opal : the 
quartz will be the product of a crystallization of the siliceous 
matter, and the opal the result of the desiccation of a gelatinous 
precipitate. I must remark, however, that this is merely a hy- 
pothesis, which, while there are some facts in favour of it, has 
also others against it; such, for example, is the existence of 
opal stalactites, with regard to which it must be admitted, that 
the matter has been in a kind of solution. 
The rocks in which opal is found sometimes (contain sulphu- 
ret of iron, in very minute crystals, which I have always found 
