540 
Oil the Food of PIrinfs. 
So far thon, the cKperimpnt succocds. but before applying; it 
extensively, it would be necessary to know more concerning the 
real effect which different ammoniacal salts produce on vegetation : 
an inquiry of great importance, and one to which the attention 
of those desirous of following out agricultural investigation should 
he particularly directed. Should the result of the inquiry be 
favourable, a few pounds of oil of vitriol, a very cheap substance, 
or hydro -chloric acid, thrown away by hundreds of tons by those 
who manufactui'e carbonate of soda from salt, or perhaps even 
common gypsum, strewed upon the dunghill or put into the 
reservoir, may be the means of getting rid of an offensive nuisance, 
and at the same time of preventing the waste of a most valuable 
substance. Night-soil might be so treated, instead of the plan 
now sometimes adopted of mixing it with lime, and thus at once 
throwing away all the ammonia. 
It must not be forgotten that although the general opinion now 
seems to be that manures for the most part act by affording 
food itself to plants, an idea has been, and still is, entertained by 
some, that certain ones at least of these substances act in the 
same manner as stimulants to animals. The best argument against 
this hypothesis is the fact that in plants nothing like a nervous 
system can be seen ; besides, when the office which each kind of 
manure fills in actually giving nourishment to a plant can be 
distinctly traced, it sc!ems unnecessary to resort to any other mode 
of explanation. 
I proceed, in accordance with the prescribed conditions of the 
essay, to offer a few remarks on certain other manures, some of 
which promise to be of value. 
Under the general term "manures'' are comprehended two 
classes of substances, namely, those which, like the animal ejec- 
tions, furnish food to plants, and those which merely improve the 
texture of the soil, and which are much better described by the 
word suggested by De CandoUe, " ameliorations." These latter 
do not require consideration ; it is perfectly evident that a stiff, 
heavy soil must be improved by an admixture of sand ; a loose 
sandy soil by clay, &c. ; but there are others, such as lime and 
burned clay, which arc more complex in their mode of action. 
Hydrate of lime, for example, appears to induce a more rapid 
decay of vegetable matter, which is sometimes, as in peaty soils, 
in excess ; when in a state of carbonate it frequently improves the 
physical condition of the land and it also enters into the plant as 
direct food, generally in combination with an organic acid : it 
may happen, although it is proljably a rare occurrence, that there 
may not exist in the soil a 'sufficient quantity of lime to serve the 
last-named purpose. Burned clay confers porosity on a heavy 
land, and probably, as Liebig remarks, performs another import- 
