148 Abbe Hauy’s Observations on Double Refraction. 
no longer communicated with the Mediterranean, or at least only 
by a channel or subterraneous passage. But this sea, which ex- 
tended at first to Hungary, has been insensibly separated from 
the Hungarian and Austrian Basin, and from that of the Rhine 
and of Switzerland ; its saline contents have continually dimi- 
nished ; it has at length become a fresh water lake ; its level has 
insensibly decreased by the deepening of the channel which con- 
veyed its waters to the sea ; and, at last, this lake has been con- 
verted into a plain with many lakes and islands. The lake has 
nearly emptied itself ; the streams of water of the different basins 
have sometimes entirely changed their direction, as in the Rhine- 
Valley ; the rivers have begun to occupy their bed nearly as at 
present ; and the original warm climate, through different phy- 
sical causes, has become temperate. Alluvial matters accumu- 
lated in Bavaria from south to north, have given rise to a great 
many of the changes in the currents of* the water at different epochs. 
Art. XIII. —Historical Account of Discoveries respecting the 
Double Refraction and Polarisation of Light. (Continued 
from Vol, viii. p. 356.) 
Sect. IV. — Account of the Experiments of the Abbe' Hauy. 
JP HE subject of Double Refraction seems to have excited al- 
most no notice during a long interval which followed the la- 
bours of Benjamin Martin. 
Towards the end of the last century, the Abbe Hauy began 
to direct his attention to the kindred subjects of Crystallogra- 
phy and Mineralogy, and to lay the foundation of that beauti- 
ful system which has immortalized his name. 
Not content with the external examination of mineral bodies, 
he directed his particular notice to all their physical properties, 
and thus availed himself of the lights of Natural Philosophy, in 
giving a scientific form to the science of minerals. 
In accomplishing this great event, he was naturally led not 
only to observe the double refraction of minerals, but to em- 
ploy it as an essential character in his descriptions ; and it was 
from this cause more than from any other, that the attention of 
philosophers was recalled to a subject which had almost disap- 
peared from modern physics. 
