RESPIRATION. 91 



from experiments, so that the interesting question, Does 

 vegetation purify or vitiate the atmosphere ? may, I think, 

 be answered in the affirmative ; it must not be denied, 

 however, that, in the opinion of many, this point is not 

 satisfactorily proved, and that if we have Confervse, Ulvse 

 and similar plants continually giving out oxygen, we 

 have others, like the Fungi, constantly absorbing it or 

 deteriorating the air in which they grow. 



83. Connected with the subject of respiration there 

 are many difficulties as yet not satisfactorily explained ; 

 the conditions under which this process is performed is 

 different in different tribes, at least as regards the lower 

 order of beings, and even the conditions under which it 

 may be carried on in the higher orders, is not, in all 

 cases, properly known. Sir Humphrey Davy found 

 some plants would grow in an atmosphere of hydrogen, 

 whilst others would not, but died. Olefiant gas, and 

 the protoxide of nitrogen, in certain proportions, are 

 not detrimental to the performance of respiration, 

 whilst the 10 1 00 of sulphurous acid gas will destroy the 

 plant. According to Saussure's latest experiments, 

 when plants were growing in the shade, any addition, 

 however small, of carbonic acid to that already existing 

 in the air, was highly deleterious, for if the air con- 

 tained a fourth part, the plant died on the sixth day, 

 but if the quantity added was small and plenty of oxy- 

 gen present, and the plant exposed to broad day-light, 

 it would live. In an atmosphere containing much oxy- 

 gen, respiration is much increased. The leaves of our 

 trees consume the greatest quantity of oxygen, and 

 evolve the most carbonic acid; then come vegetables, 

 then the leaves of Evergreens ; bog and water plants, 

 and lastly, plants having fleshy leaves. The leaves of a 

 Prunus consumed 8 parts of oxygen ; of a Potatoe 2.5; 

 of Veronica beccabunga 1.7; and of a Stapelia only 

 0. 63. 



84. The leaves of plants are undoubtedly the organs 

 by which evolution and absorption chiefly go on, whether 

 immediately connected with respiration or digestion ; 



