312 
PHYSIOLOGY. 
BOOK II. 
three similar situations, we shall find that that exposed to the 
sun has lost a great quantity of water, that in common day- 
light a less amount, and that which was in total darkness 
almost nothing." 
It is, however, to be supposed that light is in these cases 
the remote, rather than the immediate cause, of evaporation : 
we cannot apply solar light to plants without heating and 
rarefying their atmosphere, and it is the comparative dryness 
thus produced which is the great cause of evaporation or per- 
spiration. It is a well known fact that plants perspire in a 
sitting room, the air of which is constantly dry, but which is 
but imperfectly illuminated, so much more than in the open 
air exposed to the direct rays of the sun, that it is impossible 
to keep many kinds of plants alive in such a situation. 
Light is, however, to all appearance the exclusive cause of 
the decomposition of carbonic acid. It was long since re- 
marked by Priestley, that if leaves are immersed in water, and 
placed in the sun, they part with oxygen. This fact has been 
subsequently demonstrated by a great number of curious ex- 
periments, to be found in the works of Ingenhouz, Saussure, 
Senebier, and others. Saussure found that plants in cloudy 
weather, or at night, inhaled the oxygen of the surrounding 
atmosphere, but exhaled carbonic acid if they continued to 
remain in obscurity. But, as soon as they were exposed to 
the rays of the sun, they respired the oxygen they had pre- 
viously inhaled, in about the same quantity as they received it, 
and with great rapidity. Dr. Gilly found that grass leaves 
exposed to the sun in a jar for four hours produced the fol- 
lowing effect : — 
At the beginning of the experiment 
there were in the jar : — 
Of nitrogen - - - 10.507 
Of carbonic acid - - 5.7 
Of oxygen - - - 2.793 
19.000 
At the close of the experiment there 
were : — 
Of nitrogen - - - 10.507 
Of carbonic acid - - .37 
Of oxygen - - - 7.79 
18.667 
Heyne tells us that the leaves of Bryophyllum calycinum, 
in India, are acid in the morning, tasteless at noon, and bitter 
in the evening ; Link himself found that they readily stained 
litmus paper red in the morning, but scarcely produced any 
