On Rape-dust. 
185 
sorbcd by the weeds, or escapes into the air, before the young 
plant can send its fibres in search of it. An instance of the ra- 
pidity with which tlie nutritive gases escape from the soil is given 
by Dr. Madden (Prize Essay ' On the Effects of Drainage,' 
'Quarterly Journal of Agriculture,' Dec. 1841). He passed a 
stream of amynonia for an hour into dry soil ; upon weighing it, 
he found that it had retained only I ^ per cent, of its own weight 
of gas. 198'6 grains held 3 grains of ammonia. The soil was 
then wetted and exposed to the atmosphere, and in three hours it 
was found to have lost 1'8 grains of the gas. 
Again, carbonic acid gas was passed over 207"2 grains of soil 
for an hour, when it was found not to have increased in weight 
SLifiiciently to be appreciated ; it had not retained per cent, of 
carbonic acid. 
By this it is evident that from a rapidly decomposing substance 
much of the nutritive gases must escape, if they are evolved at a 
distance from the plant.* 
And if both the rape dust and the seed are spread by the hand, 
there is a double ivaste — a waste of rape-dust, owing to the extent 
of surface covered by it, and to the escape of its gases into the air, 
and the absorption by weeds (which cannot be destroyed until the 
after-harvest) — and also a waste of seed, to the extent of, at the 
* Cuthbert Johnson, who has laboured zealously in advocacy of the 
drill system, gives " a chemical reason why the manure drill should be 
adopted to bring, as closely as possible, every plant into immediate con- 
tact with the decomposing manure he applies to the soil, and that is, the 
superior readiness with which, in all cases of decomposition, the disen- 
gaged substance enters into new combinations at the very instant of its 
disengagement, than it does after it has been completely formed. Thus, 
to give an instance, during the putrefactive fermentation of vegetable sub- 
stances a quantity of nitrogen is disengaged, and if this takes place under 
certain favourable circumstances — such as the presence of calcareous 
matters, potash, and a dry warm temperature at the moment it is formed 
— the nitrogen combines with oxygen, forms nitric acid, which unites 
witli potash, thus nitrate of potash, saltpetre, is formed ; but if the 
nitrogen is once fairly disengaged, almost every endeavour of the chemist 
has failed in making it unite with oxygen so as to form the acid of salt- 
petre." — Encyclopccdia, p. 538. 
However open to doubt this theory of the formation of nitric acid by 
the decomposition of ammonia and the combination of its nitrogen with 
the oxygen of the air may be considered, the tendency which elementary 
substances have to unite with each other when in their nascent state may 
be proved easily. Thus the formation of ammonia by the union of hy- 
drogen and the nitrogen of animal and vegetable decompositions can 
be proved by different processes. Professor Johnston {Lectures, part i. 
238) gives several experiments illustrative of this union, and of the 
general principle above named. 
