522 
On the Fvod of Plants. 
under favourable circumstances this gas can combine directly with 
oxvgcn and with carbon, even in our hands ; and surely the; 
chemical energies at work in a living plant are, to say the least, 
equal in power to those which we have under our control in the 
laboratory. Nevertheless, in the present state of our knowledge, 
it seems that we must look to ammonia as the general source of 
the azote found in plants. 
In Boussingault s paper already referred to an Interesting 
account will be found of certain experiments on the cultivation of 
Jerusalem artichokes in Alsace, in which a prodigious quantity of 
nitrogen is taken up from the air. This culture is carried on con- 
tiimonshj, the ground being manured every two years. A crop of 
23,550 lbs. per acre is obtained from a somewhat shallow soil at 
Bechelbronn, which is shown to contain no less than 38 '3 lbs. 
more nitrogen than the manure applied, and which must have 
been derived from the air. It seems difficult at first to imagine 
that this large quantity could have been got from the very minute 
trace of ammonia present in the atmosphere ; but a little con- 
sideration will show that even in this case there is no necessity to 
suppose an absorption of gaseous nitrogen. 
If, taking Liebig s data, we assume one pound of rain-water to 
contain not more than one-fourth of a grain of ammonia ; and the 
annual fall of rain to amount in round numbers to 4,000,000 lbs. 
per acre,* this quantity of water will bring down with it in the 
state of ammonia no less than 1 17'5 lbs. of nitrogen. 
Such then are the conclusions to which we come at the present 
moment. Setting aside certain mineral matters furnished by the 
soil, which will be discussed immediately, the only substances 
really required as food by living plants are carbonic acid, water, 
and ammonia — tlircc inorganic bodies ; and that manures owe a 
great part of their value to the supply of these necessary ele- 
ments, which they furnish by their decomposition. Vegetables 
are thus machines for manufacturing out of these few and simple 
substances the complicated products, alljumen, fibrin, casein, 
starch, sugar, &c., which animals require for their sustenance, 
while the bodies of the latter when witlidrawn from the influence 
of life, and once more brought under the sole dominion of mei'e 
chemical agencies, furnish by their decomposition the food re- 
quired for another generation of plants. 
* Under the mark. The mean of twenty-two years' observations at 
Paris — a drier climate than tliat of Ensjland — gives for the annual fall of 
rain 57 centimetres = 22-44 inches. This makes 3240 cubic inches per 
square foot, weighing 117 lbs. nearly. The Enj^lish acre contains 4;J,5tiU 
square feet, consequently the weight of the rain per acre will be 435ti(J X 
117 = 5,096,520 lbs. yearly. 
