On Rape- dust. 
181 
cm flourish — without <a due supply of atmospheric moisture. 
The necessity for a supjily of oxygen and hydrogen explains this. 
To manures of an oily nature, this supply of moisture, experience, 
as in the case of rape-dust, has proved to be highly necessary. 
In explanation, it should be remembered that carbonic acid, 
evolved during vegetable decomposition — that ammonia, given off 
during the decay of animal matter — and the carbonate of ammonia, 
formed by the combination of these two gases, are all soluble ; 
and also that plants absorb a large proportion of these gases (car- 
bonic acid and ammonia) in a state of solution, in which condition 
they lose much of their tendency to escape in the gaseous form ; 
hence, during showery seasons, much of the ammonia and car- 
bonic acid of rape-dust, which in dry weather rises into the 
atmosphere, is washed to the I'oot of the plant ; which, owing to 
its liberal supply of moisture, is in a healthy condition ; and can, 
consequently, assimilate a larger portion of food. In drought the 
reverse is the case ; being poorly supplied with water, the plant 
languishes, and can appropriate but a slight portion of the nutri- 
tive gases, which, when not in solution or combination, quickly 
escape. 
3rd. That rape-dust is most certain in its effects when applied 
to the winter-sown wheat is explained by the fact, that there is 
never any dejjciency of moisture, which it requires, as we have 
seen already, and that it acts quickly and pushes on the growth 
of the young plant ; by which means it is better able to bear the 
severity of winter. That in favourable seasons — that is, when 
the germinating seed is supplied with moistvre (a circumstance 
which is uncertain) — the return for the application of rape-dust 
should be most remunerative upon spring crops, is explained by 
the fact, that as there is no cessation of vegetation after the plant 
appears, the nutritive gases are assimilated as soon as they are 
given off. And as there is no waste of these, as in wheat during 
wintei*, when the plant cannot appropriate it, less tillage has an 
equal effect — equal tillage a greater. This, of course, depends 
entirely on the season. 
4th. That rape-dust should succeed better for the wheat crop 
upon strong land is explained by the fact that the soil is not so 
permeable to the atmosphere as that of a sandy or calcareous na- 
ture ; hence a much less portion of the carbonic and ammoniacal 
gases of the tillage escape during winter, and at the same time 
the active putrefactive fermentation of the rape-dust causes a 
disintegration or pulverization of the soil round the roots of the 
gypsum, the great difSculty with which it is dissolved explains the fact. 
— 77je Author. 
