On tho Food of Plants. 
521 
portion, is in .a state of abnormal development — a state excellent 
lor the purposes lor whicli the plant is designed by man, but still 
a forced and unnatural state ; the case is changed, and without 
an artificial supply of this element in an available form, the plants 
languish and perhaps die. 
Hence, oiio of the great uses, but not the sole use, of animal 
manure, more especially of urine, whose truly wonderful effects 
have been observed by many. Putrid urine is probably the most 
valuable manure we possess. The people of the Low Countries 
and China are well aware of this. It should be collected and 
husbanded with the greatest care in our towns and cities, which 
could easily be done if public attention were once directed to this 
important subject, f(n" the use of the farmer, who cannot value it 
too highly. 
The mode in which this manure acts is quite intelligible, and 
gives great support to the supposed function of ammonia : — fresh 
urine contains some 3 per cent, of a white, soluble, crystallizable 
substance called "urea," which on contact with water and the 
mucous matter contained in the urine undergoes, under the in- 
fluence of this mucus (which acts towards it in all probability in 
the same manner as yeast does towards sugar), a peculiar change 
or fermentation, whereby it becomes converted into carbonate of 
ammonia, and in that state is taken up and assimilated by the 
plant. It furnishes, in short, the same substance as the at- 
mosphere, the food of plants provided by Nature herself. More- 
over, urine contains a large quantity of phosphates, bodies we shall 
hereafter see indis^iensable to vegetable life. 
If the proposition discussed be true, it follows that the exist- 
ence of ammonia in the air must have preceded both vegetable 
and animal life ; that substance must have been an original con- 
stituent of our globe, and is not all due to putrefaction. 
We are destitute of data at present to determine the point 
whether or not gaseous nitrogen can be fixed in a growing point, 
and in the absence of such we must not hastily decide that such a 
thing never happens. It is perfectly true that this substance 
manifests under ordinary circumstances very little tendency to 
combine with other bodies, yet we know of two exceptions to 
this rule, and a diligent search would probably be rewarded by 
the discovery of others. When nitrogen is mixed with excess of 
hydrogen, and burned at a jet in oxygen gas, water and nitric acid 
are produced ; and secondly, when pure nitrogen is passed over a 
mixture of charcoal-powder, and carbonate of potash, cyanide of 
potassium in quantity makes its appearance.* Thus we see that 
* Journal of the Pharmaceutical Society of London, Jan. 1842. 
