252 The Atmosphere as a source of Nitrogen to Plants. 
constitutes so very important a class of all organized structures — 
a class absolutely indispensable to the existence on the earth of 
animal life. 
Tlie oljject of these pages is to review as fully, and yet as suc- 
cinctly as possible, in the first place, the various investigations 
vifhich have been made, with the view of determining this point; 
and secondly, to call attention to the progress which has, more 
especially of late, been made in discovering and estimating at 
their real value other atmospheric sources of nitrogen, in lieu of 
the great natural supply, supposing it to be found inadequate to 
the necessities of the case. It would, however, be found very 
inconvenient to follow these subjects separately, and I shall there- 
fore take them very much in the order of time as the experiments 
were recorded by their different observers. 
Upon the discovery of the composition of the air, it became 
natural tliat attention should be drawn to the circumstances under 
which plants vegetate in it. Priestley, to whose researches upon 
the chemistry of the gases so much of our earlier knowledge was 
due, believed that he had found that when plants growing in water 
were placed in a confined portion of air, they had the effect after 
a time of reducing the bulk of it very considerably. He con- 
sidered that in these circumstances the diminution of volume was 
due to an absorption by the plant of the atmospheric nitrogen ; and 
other chemists (Ingenhouz, Sennebier, &c.), by whom his experi- 
ments were repeated, came to the same conclusion. These views, 
however, were opposed by Saussure, whose experiments on the 
chemistry of vegetation are far more trustworthy than those of 
his predecessors. After repeated attempts with the methods 
adopted by Priestley, he failed to observe any absorption of 
atmospheric nitrogen by plants. Saussure came to the conclusion 
that the nitrogen of plants could only be derived from the veget- 
able and animal matters diffused through the soil, or existing in 
the form of ammoniacal vapours in the air and brought down by 
rain. That such ammoniacal vapours do exist in the air Saussure 
considered to be proved by the change occuning to sulphate of 
alumina, which, when left exposed to the air, becomes by degrees 
converted into double sulphate of alumina and ammonia. 
To Saussure therefore belongs the credit of having first, although 
in a very general way, suggested ammonia in the air as the pro- 
bable source of the nitrogen of plants. We shall presently see 
how far such an explanation is sufficient to account for the 
observed phenomena. The earlier experimenters upon these 
interest, and upon which much difference of opinion exists — we may, for our 
present purpose, admit the sufficiency of this supply for the chief function of 
plants, namely, the preservation of their species. 
