64 
POPULAR SCIENCE REVIEW. 
which have been performed in this direction, but also by 
analogy ; no free element, save perhaps oxygen, being assimi- 
lable. Ammonia, as well as nitric acid and nitrous acid, is 
however present in rain, river, and well waters, more or less 
constantly ; but then we know that ammonia in the soil is 
rapidly oxidised, and that the conditions to which it is there 
subjected are such that it can hardly reach the plant in an 
unoxidised state. The total atmospheric nitrogen compounds 
reaching the earth in rain is very variously estimated, and 
seems to be by no means a constant quantity. But it must be 
remembered that dew and mist and fog, as well as the atmo- 
sphere itself, contain the three nitrogen compounds before 
mentioned, and that no attempt has been made to estimate 
simultaneously all the atmospheric nitrogen compounds when- 
ever occurring, for a whole year : the difficulties of the task are 
indeed enormous. But until we have further information on 
this point it would be rash to affirm that our supplies of nitro- 
genous atmospheric compounds are always more than sufficient 
for the normal and natural condition of vegetation where 
nothing is finally removed from the soil. One thing we know : 
that an increased supply of nitrogen is often most effective in 
promoting vegetation, more conspicuously rapid and effective 
indeed than the proportionate addition of any manurial matter, 
even when all the other conditions of the experiment are 
suitably adjusted. Kecent experiments have moreover shown 
that whatever may be the form in which nitrogen is directly 
assimilated, indirectly plants may derive it, not only from 
nitrous acid, and of course from nitrites, but also from ammonia 
and from most nitrogenous compounds. Uric acid, urea, and 
gelatine, can all contribute it with nearly equal ease and rapidity 
to the living plant. Even wool,, so difficult to alter, so insus- 
ceptible of change, yet gives up, in course of time, its nitrogen 
in an assimilable form. One important difference between 
ammonia and nitrates ought to be here noted ; ammonia, as long 
as it remains unoxidised, is retained in ordinary soils very firmly ; 
nitrates are not. Then too, soon after a field, say of wheat, has 
been top-dressed ” in the spring with ammonic sulphate, a 
marked increase in the amount of nitrates in the drainage water 
of the field has been observed ; a circumstance pointing at once 
to the ready oxidisability of ammonia and to the ease with which 
nitrates (chiefly calcic nitrate) are washed out of the soil. 
We turn now to the ash, or non-volatile constituents of the 
plant. Of the oxygen, the sulphur, and the carbon found in 
part in the burnt residue of vegetables, we have already 
spoken, but among the non-metallic ash elements there remain 
four still to be considered : phosphorus, silicon, chlorine, and 
fluorine. The singular importance of phosphorus among these 
