fying the air. Our scientific knowledge, however, should not, 

 as it is often liable to do, get the better of our " sense uncom- 

 mon, men call common sense," for the brightness and cheer that 

 flowers bring to the sick need never be sacrificed for fear of their 

 evil effects upon the air. 



The discovery and elucidation of plant respiration was one of 

 the most, if not the most, important contributions ever made to 

 the science of plant physiology. It throws a flood of light upon 

 metabolism, and in metabolism is locked up the secret of secrets, 

 whose finding out is the ultimate problem of all biology, viz., 

 the answer to the question, What is life ? It is fitting, therefore, 

 that we should know something of those masters of experimental 





are indebted for what is now known of the subject. 



Since the process involves an understanding of the relation 

 between plants and air, it is obvious that it could never be under- 

 stood until the nature and properties of air were clearly compre- 

 hended. On this question we are all familiar with the fantastic 

 notions of antiquity. Thales, of Miletus, had taught that all 

 things were made from water, but Anaximenes, his fellow towns- 

 man, declared that everything is made of air. And, since it is 

 the air that gives his life to man, it must be his very soul. From 

 this it was justly inferred that the infinite air was God, and that 

 it is the source of all the gods and goddesses. 



Diogenes, of Apollonia, went a step farther, and said that the 

 whole world is a living being. Air is not only the soul of man, 

 but also the soul of the world. By an ingenious logic, he reasoned 

 that air " knows much." " But that which has knowledge," said 

 .he, "is that which men call air; it is it that regulates and governs 

 all, and hence it is the use of air to pervade all, and to dispose 

 all, and to be in all, for there is nothing that has not part in it." * 

 How surprised he would be to-day to find how near he came to 

 expressing the truth ! Since, said Diogenes, plants have no air 

 cavities, and since they are wholly unintelligent, the intelligence 

 of man is due to the flowing of air through his body in the 



