460 EXTENT OF SOIL IRRIGATED IN EUROPE. 
As near as can be ascertained, the amount of water applied 
to irrigated lands is scarcely anywhere less than the total pre- 
cipitation during the season of vegetable growth, and in gen- 
this remark, as it seems almost impossible to apply too much water to them, 
provided it be kept in motion and not allowed to stagnate on the surface. 
Professor Liebig, in his Modern Agriculture, says: ‘‘ There is not to be 
found in chemistry a more wonderful phenomenon, one which more con- 
founds all human wisdom, than is presented by the soil of a garden or field. 
By the simplest experiment, any one may satisfy himself that rain-water 
filtered through field or garden soil does not dissolve out a trace of potash, 
silicic acid, ammonia, or phosphoric acid. The soil does not give up to the 
water one particle of the food of plants which it contains. The most con- 
tinuous rains cannot remove from the field, except mechanically, any of the 
essential constituents of its fertility.” 
‘* The soil not only retains firmly all the food of plants which is actually in 
it, but its power to preserve all that may be useful to them extends much far- 
ther. If rain or other water holding in solution ammonia, potash, and phos- 
phoric and silicic acids, be brought in contact with soil, these substances dis- 
appear almost immediately from the solution; the soil withdraws them from 
the water. Only such substances are completely withdrawn by the soil as are 
indispensable articles of food for plants; all others remain wholly or in part 
in solution.” 
These opinions were confirmed, soon after their promulgation, by the experi- 
mental researches of other chemists, but are now questioned, and they are not 
strictly in accordance with the alleged experience of agriculturists in those 
parts of Italy where irrigation is most successfully applied. They believe 
that the constituents of vegetable growth are washed out of the soil by exces- 
sive and long-continued watering. They consider it also established as a fact 
of observation, that water which has flowed through or over rich ground is 
more valuable for irrigation than water from the same source, which has not 
been impregnated with fertilizing substances by passing through soils contain- 
ing them; and, on the other hand, that water, rich in the elements of vege- 
tation, parts with them in serving to irrigate a poor soil, and is therefore less 
valuable as a fertilizer of lower grounds to which it may afterward be con- 
ducted. See BarrD Smiru, Ltalian Lrrigation, i., p. 25; Scorr MONCRIEFF, 
Irrigation in Southern Hurope, pp. 34, 87, 89 ; LOMBARDINI, Sulle Inondaziont, 
etc., p. 73; Mancon, Les Irrigations, p. 48. 
The practice of irrigation—except in mountainous countries where springs 
and rivulets are numerous—-is attended with very serious economical, social, 
and political evils. The construction of canals and their immensely ramified 
branches, and the grading and scarping of the ground to be watered, are 
always expensive operations, and they very often require an amount of capital 
which can be commanded only by the state, by moneyed corporations, or by 
very wealthy proprietors ; the capacity of the canals must be calculated with 
