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- 

 spii'ation. 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. Tliis fact has been 

 subsequently demonstrated by a great nvimber 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 cai'bonic 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 Svere 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 

 Ofoxvffen ... 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 



