TO THE PASSAGE OF MOUNT HiEMUS. 195 
atmosphere, there are none so remarkable as chap. 
those which result from the decomposition of • 
the hydrates oi Silica; the whole beauty of the 
noble opal, and all that constitutes its distinction 
from semi-opal, being due to such decomposition. 
The supposed opal, discovered at Alexandria in 
Egypt, certainly owed its remarkable properties 
to the circumstance of its having remained for 
such a length of time exposed among the ruins 
of the city ; as it proved to be a mass of glass ^ 
a substance extremely liable to exhibit opales- 
cence, when decomposed by weathering. 
Afterwards, we called at the house of the 
celebrated engineer Kauffer, whose name is 
often mentioned in books of travels*. He re- 
ceived us upon his death-bed, putting into our 
hands the valuable maps, which had been 
already engraved, from his surveys of the Topo- 
graphy of Constantinople and the Sea of Marmora. 
We received his permission to publish them upon 
a reduced scale in England; and they are now 
added to this volume. He regretted that he 
could not shew to us many original and valu- 
able papers belonging to his collection, owing 
(3) See the French Encydoptdie, article Opale, torn. II. p. 493. 
Neufehast. 1765. 
(4) See particularly the " Voyage de la Propontide et da Pont 
Eu3>m, par J. B. Le Chevalier, &c. tern. II. p. 172. Parh, 1802. 
O 2 
