77 
liberating its oxygen; that this operation was only carried on during 
the day, but that at night the function was reversed, the oxygen 
absorbed and returned in the form of carbonic acid. That was the 
statement which students were expected to receive without explanation 
or comment, yet, if true, it involved one of the most remarkable 
phenomena in the whole circle of natural science. 
The direct decomposition of a compound was an operation which 
chemists but seldom performed, and which presented great difficulties 
when the affinity between the constituent elements was great. He 
might safely say that a means of directly decomposing carbonic acid 
was yet unknown to chemical science. The animal organisms gave 
facilities for chemical union, as in the lungs ; for solution and precipita- 
tion, as in the stomach and duodenum ; and they determined the re- 
arrangement of organic compounds in the various glands ; but we had 
yet to learn that avect decomposition was among the animal functions. 
Yet we were told that the mere cuticle of a leaf was able to do that 
which the whole animal organism seemed unable to accomplish, and 
which human science, with all its appliances, had not succeeded in 
imitating ; while the same cuticle was credited with the contradictory 
property of performing, in the cold, and without special apparatus, the 
same function which the complicated structure of the lungs only 
discharged at a very high temperature. 
But when the puzzled student sought for higher information in 
those treatises which were avowedly the treasuries of matured 
knowledge, he found not only that the explanations given were in- 
sufficient and variable, but that the very facts themselves were called 
in question, or confronted with others equally incomprehensible and 
self-contradictory. 
Thus Ingenhousz, Priestley, and Percival narrated experiments 
performed on plants immersed in an atmosphere so largely charged 
with carbonic acid as to extinguish a lighted candle, the result of which 
was that the plant throve, and purified the air in three hours. Yet 
Daubeny asserted, on the strength of most careful experiments, that 
plants exposed to an atmosphere containing 20 per cent. of carbonic 
acid, died in a few days, even though exposed to direct sunlight. 
Ellis affirmed with the concurrence of Burnett, Garreau, and Carpen- 
ter, that plants exhaled carbonic acid at all times though sunlight re- 
decomposed a portion. Bloxham, on the chemistry of plants, asserted 
that leaves decomposed carbonic acid by day, and exhaled it by night 
to a less extent. Pepys, Cloez, and Gratiolet denied that any action 
whatever took place in darkness, but affirmed the disengagement of 
