ks natural situation, but places it in circumstances 
where the external air is entirely excluued. The 
existence of aquatic plants under water is no pjoof 
that terrestrial ones are able to support a condition 
so unnatural to them ; tor the structure in each may 
be very different which enables them to live and 
flourish in situations so very dissimilar. Thus we 
know that the leaves of the ranunculus aquaiicus, 
which float on the surface of the water, have a flat 
and expanded form, while those, which are entire- 
ly immersed, exhibit a roundish and filamentous 
figure. 
296. But it is another objection to these experi- 
ments, that they bring many new sources of fallacy 
into view, which greatly embarrass the original ques- 
tion ; since it is not easy to determine how much of 
the air is furnished by the water itself, and how 
much by the vegetable immersed in it. These, accor- 
dingly, have been points much disputed among those 
who have made experiments on the subject. We pre- 
fer, therefore, those experiments of Dr Ingenhousz 
which we have before detailed (290. 1.), as affording 
the best evidence adduced by him of the purification 
of the atmosphere, by plants exposed to the sun. 
297. In these experiments, the air, vitiated by re- 
spiration and combustion, appears always to have con- 
tained carbonic acid, at least no notice is given of 
this acid having been previously removed. Farther, 
Dr Ingenhousz supposed plants to possess the singu- 
lar property of converting carbonic acid into respira- 
ble air, when exposed to the sun ; and he asserts, 
that if they are confined in mixtures of common air 
E2 
