65 
time only after the sun has gotten above the horizon, 
is more or less vigorous in proportion to the clearness 
of the day, and the situation of the plant to receive 
the direct rays of the sun, and is suspended entirely 
during the darkness ot the night ; that plants, in the 
shade, do not ameliorate the air, but render it more 
noxious; that the leaves, and green branches and stems 
of plants alone perform this office of purification ; and 
that acrid, offensive, and poisonous plants act in the 
same manner as the most sweet and salutary ; that 
the pure air proceeds chiefly from the inferior sur- 
face of the leaves, and that young leaves do not fur- 
nish so much of it as those which have acqui- 
red their full size and vigour ; that some plants yield 
purer air than others, and that aquatic plants par- 
ticularly excel in this respect ; and, lastly, that the 
sun does not possess the power of improving the qua- 
lities of atmospheric air without the concurrence of 
plants. These conclusions were drawn from the re- 
sults of more than 500 experiments, made between 
the months of June and September *. 
293. Of this great number of experiments, how- 
ever, almost all were made with the leaves after they 
were separated from the branches, and immersed in 
glass vessels of water. Dr Ingenhousz says, that 
this mutilation does not vary the result ; for when he 
passed the pliant branches of vegetables, whose roots 
remained in the earth, into jars of water, the same 
phenomena were exhibited ; and therefore, he after- 
wards used the leaves and cut branches as being most 
convenient f. And with respect to immersion in wa- 
* Exper. t. i. pref. p. 64. et seq. f Ibid. p. 283. 
