Dr Giiby on the Rcspiratiorh (^Plants. 103 
It is here fit that I should advert to a circumstance which is 
evident from the tabular statement I have given. It will be 
seen, both in that, and in the one before it, that the proportion 
of nitrogen to the other constituents, was greater at the clbse 
than at the commencement of the experiment. This, however, 
is a complete illusion. There is in fact no increase of nitrogen. 
I am quite clear, that the apparent addition of it arises from 
some portion of carbonic acid, Avhich the sun has not had time 
or power to decompose, being retained in the leaves, and in this 
way lessening the absolute quantity of air in the phial ; so that 
the more carbonic acid is retained in the leaves, the greater will 
be the ratio of nitrogen. To make this perfectly clear : — In the 
experiment above related, if all the carbonic acid, with the excep- 
tion of 19.5 (the quantity remaining in the red phial) had been 
decomposed, we ought certainly to have found 35.47 of oxygen ; 
but it appears that only 29-5 w^ere indicated by the eudiometer, 
and therefore, we cannot hesitate in admitting, that 6 per cent, 
were retained in the leaves. The result of one or two trials 
would not make me so confident as to this conclusion ; but ha- 
ving invariably found, from a very frequent repetition of my 
experiments, that the carbonic acid that disappears is not ac- 
counted for in the oxygen formed, I think the inference which 
I have drawn is perfectly fair, and indeed unavoidable. This 
apparent accession of nitrogen is noticed by Saussure, in his ad- 
mirable work entitled Kecherches, &c. as a real production of 
that gas ; but I conceive that the circumstance is much more 
naturally accounted for by the explanation I have just given. 
If this explanation be correct, it will serve to shew, that this 
decomposition of carbonic acid takes place within the sub- 
stance of the leaf, and not exterior to it, as Mr Ellis ima- 
gines. 
I shall now proceed with the chief object of this paper, to in- 
quire how far the respiration of plants is conducive to the puri- 
fication of the atmosphere. As far as is known, from the expe- 
riments and observations of the ablest inquirers, it appears, that 
plants in darkness, in the shade, and perhaps in the common 
light of the day, generate carbonic acid, and that only during 
sunshine is there any evolution of oxygen. The question, there- 
fore, is, Has this partial production of oxygen any ej0Pect in dk- 
