ill ?/.<f relation to A(jricnltnre. 
429 
ultimate appropriation of rain-water is evident ; but tlic modes 
and limits of their action arc still to be determined, and when 
this is- done a c;reat service will have been rendered to agri- 
culture. However, it is but just to say that French savants, 
stimulated by the initiative of the Government, who have at 
their command an admirably organised body of engineers, under 
the name of the Adiniimtration dcs Fonts et Chaussecs, have gone 
far towards the final elucidation of many important hydrological 
phenomena presented by the various streams of the French terri- 
tory. The records of these interesting researches have lately been 
collected into a volume, called the ' Annuaire des Eaux de la 
France,' published by the Frencli Government, a copy of which 
lias been kindly lent me by the Department of Agriculture, to 
which 1 am further indebted for many other valuable works not 
easily obtained from any other sources. 
Rain-water, when collected just before its contact with the 
surface of the soil, and therefore before its nature can be modi- 
fied by the solution of any earthy substances, has been found to 
contain, in various proportions, some gaseous acids, such as azotic 
;icid, free or combined with ammonia, nitric acid, and even traces 
of iodine, which account for its powerful action on growing crops, 
especially after a thunder-shower. But it is principally after its 
contact with the soil in its passage through soluble geological 
strata, that its physical nature undergoes the most conspicuous 
changes. 
The physical nature of the waters of a river will greatly depend 
on that of the geological strata with which its source and that of 
Its affluent tributaries are in contact ; I say its own source, 
because it is found that all large rivers, for a considerable dis- 
tance before they reach the sea, run in channels scooped out of 
tertiary alluvial deposits, the nature of which is identical with 
that of the alluvial earthy matters brought by the affluent streams, 
and dej)ositcd in the main channel by precipitation. But in the 
case of long rivers, the nature of the strata washed by the tribu- 
tary streams often varies exceedingly ; hence the complex consti- 
tution of the silt of many rivers in the lower reaches of their 
course ; but as the chemical condition of the tributary streams is 
generally homogeneous, and consequently forms a characteristic 
feature of its water, it is interesting to note the change that the 
waters of the main stream undergo after they have received the 
tribute of their principal affluents. Thus it is found in the 
basin of the Seine that the waters from the crystalline soil of the 
Department of Yonne, conveyed to the Seine through the river 
Yonne, contain a notable quantity of alkaline silicates ; whilst 
those of Arceuil are essentially calcareous, and those flowing 
through the gypsum strata of Belleville and Menilmontant, in the 
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