536 M. Beudant ou the Opals Hungary. 
rare to find possessed of the requisite beauty, because the reflec- 
tions are destroyed by the mixture : it is also much esteemed on 
account of its rarity ; the reflections are dull, and most com- 
monly violet or purple, all the other tints being absorbed by the 
colour of the basis. 
It is the case that opals of all kinds are susceptible of altera- 
tion ; they become white, lose their hardness, and may be 
scratched with the point of a knife ; they adhere strongly to the 
tongue, imbibe water, and, by this means, sometimes regain 
their transparency and iridescent colours. It is in this state 
that they have been named Hydrophane ; but under this appel- 
lation true opals, in an altered state, have been united with cal- 
cedonies equally decomposed, and which also regain, when 
placed in water, more or less of their transparency. 
On subjecting these altered opals to various experiments, I 
found that they also regain their transparence, when heated 
strongly before the blowpipe ; they crack (or split), more or less, 
at the same time, which seems to indicate a condensation of the 
substance, and announces, in a different way from that hitherto 
known, that the opacity is owing to the vacuities contained in 
the stone. Another observation, which I made at the same time, 
is, that the opaque varieties, which resume their iridescent co- 
lours after immersion in water, do not regain them from the ac- 
tion of fire ; they merely become transparent. 
Sometimes the alteration and decomposition of opal become 
complete, and there results a white substance, perfectly earthy, 
and capable of mixing with water, but not, however, so as to 
form a paste like argil. This earth is sometimes carried away 
by the water, and deposited in the cavities which occur in its 
course'. 
Independently of these alterations, there exists also, in the 
most solid and freshest parts of the rock, small nests of a very 
soft matter, which readily cuts, and produces a particular unc- 
tuosity under the edge of the knife. This matter is whitish, 
yellowish, bluish, and sometimes it presents indications of irides- 
cent reflections. It is very soft to the touch, and, when it has 
imbibed water, becomes sufficiently tenacious to be kneaded be- 
tween the fingers. I cannot believe that this matter is owing to 
a decomposition of opal, similar to that which we have just men- 
