Nov. 1896.] BOTANICAL SOCIETY OF EDINBURGH 3 



vapour usually forming less than one but often more than 

 two per cent, of the air, and carbonic acid present to the 

 extent of about three or four parts in ten thousand. Still 

 more elaborate analyses have shown that ammonia is in 

 the air to the amount of one part per million, or less, and 

 that traces of nitric acid are also sometimes to be found. 



The relation of these gases to plant life very soon began to 

 be noticed, speculated about, and experimentally investigated. 



Priestley, the year before he discovered oxygen, had, 

 with the watchful eye of genius, made a very interesting 

 observation. He found that air which had been "depraved," 

 as he called it, by burning a lamp in it, or by breathing in 

 it, could be restored to its former purity by putting a 

 growing plant in it and exposing it to the sunlight. 



The explanation of that curious circumstance did not 

 come till a good while later. Indeed, it was not till the 

 beginning of this century that botanists were assured that 

 plants with green leaves took their carbon from the carbonic 

 acid of the air, and gave out a corresponding quantity of 

 oxygen, and that one of the great functions of plant life was 

 the restoration to the atmosphere of the oxygen of which 

 it had been bereft in the universal processes of respiration 

 and combustion. 



Familiar as we are with that fact at the present day, it 

 never ceases, and never can cease, to be a subject of great 

 interest and continual wonder that the green vegetation 

 that clothes the globe, from the tiniest alga to the greatest 

 forest tree, derives the half of the dry matter of which it 

 is composed, viz. its carbon, from the carbonic acid of the 

 air, although that gas is present in the atmosphere only to 

 the limited extent above mentioned. It w\as found that 

 this formative process, which is called assimilation, went 

 on only in daylight, and most vigorously in sunshine. 

 During the night or in the dark, plants were found to give 

 out only carbonic acid, and further investigation showed 

 that plants were constantly giving out carbonic acid both 

 by night and by day in common with all other organised 

 beings, whether vegetable or animal, and that act is 

 known as respiration. Every living thing must breathe ; 

 it must take in oxygen to burn up its waste carbonaceous 

 matter, and give it out as carbonic acid. 



