430 
Tlie Water Economy of France 
nei<rhbourho()d of Paris, are strongly impregnated with sulphate 
of lime. But many of these mineral substances are not soluble in 
pure water, and therefore it is evident that the water must 
contain some dissolving agent which acts upon the various strata, 
and allows it to carry away in its course a portion of the sub- 
stances over which they flow. This agent is found to be chiefly 
carbonic acid ; and it is to the presence of a large quantity of 
that acid in water flowing through calcareous strata, that such 
water possesses the property of incrustation, choking the pipes 
through which it may flow, dripping into fantastical stalactites 
in limestone caves, and coating every object with which it is 
brought in contact with a covering of solid stony matter. Other 
waters are known to deposit a kind of ferrugineous coating when 
in contact with iron, and completely choke the pipes through 
which they are made to flow. There are other substances which 
impart peculiar medicinal virtues to mineral waters, which are 
well known, but foreign to the objects of this paper. 
No circumstance in the course of a river influences more the 
physical nature of its waters, than its passage through a large 
city. The amount of sewerage it receives must not only add to 
its constituents a vast amount of new organic elements, but these 
new ingredients must produce, and themselves undergo various 
chemical changes on being brought into contact with other 
substances already contained in the stream. Some very interest- 
ing researches have been made in France on this important 
subject, particularly by MM. Boutron and Henry, in respect to 
the river Seine above and below Paris ; and by MM. Girardin 
and Preisser, on the precipitation of the mineral salts held by the 
water of rivers at various points of their courses, and upon the 
causes by which such precipitations are determined. But before 
I come to these interesting details I will briefly describe the 
g-eolosjical characteristics of the four ffreat water divisions of 
France, taking the divisions in the order in which I have 
noticed them. 
In the north-eastern basin the small French tributary streams 
of the Scheldt, the Scarpe, which passes through Arras and 
Douai, and the Lys, which flows at a short distance from Lille, 
may be dismissed with the remark that they both come from the 
chalk protuberance, which begins at Cape Grisnez, and, after 
having reached the base of that formation, flow over the nearly- 
level tertiary deposits and alluvial plains of remarkable fertility, 
which form the French Flanders. 
The river Meuse calls for more distinct notice. This river takes 
its source from the south-western angle of the Vosges, near the 
Langres plateau, and flows as far as Mezieres in a valley scooped 
out of the oolitic formation. It runs first in a north-western 
