THE LEAF AS A PHYSICAL STUDY. 139 



that, at the epoch of the growth of those enormous primeval forests 

 which supplied the material of the coal formation, the atmosphere 

 was highl}' charged with carbonic acid, as well as with humidity ; 

 and that from this source the ferns, lycopodiaceae, and coniferse 

 of that era were enabled to attain their gigantic proportions. He 

 imagines that they not only thus converted into organized products 

 an immense amount of carbonic acid which had been previously 

 liberated b}- some change in the mineral world, but that by 

 removing it from the atmosphere they prepared the earth for the 

 residence of a higher class of animals. It is now regarded by 

 scientists as a fixed fact that the whole vast accumulation of carbon 

 at present in the earth was at one time a component part of the 

 atmosphere, and recent investigations and experiments justify the 

 assertions of M. Brongniart. Five parts in ten thousand is the 

 amount of carbonic acid now contained in our atmosphere, and this 

 seems to be in general as much as plants require ; but it is not at 

 all improbable that in those early epochs a higher percentage may 

 have existed, for we know by direct experiment that plants will 

 thrive in an atmosphere of from five to eight per cent of carbonic 

 acid, provided they are supplied with strong sunlight; and we 

 know that in those regions where a larger percentage does exist 

 plants seem to thrive with increased vigor. Tlius, in the Lake 

 Solfatara, in Italy, where carbonic acid gas escapes from the bottom 

 with such violence as to give the appearance of ebullition, floating 

 islands are constantl}' being formed, which consist of confervse 

 and other simple cellular plants. The celebrated springs of Got- 

 tingen, which abound in carbonic acid, are another example ; for 

 there, also, we find the surrounding vegetation of increased luxu- 

 riance ; appearing earlier in spring, and continuing later in autumn. 

 So it may be that the leaf rendered our atmosphere respirable, 

 as well as keeps it so today, by the removal of an ingredient so 

 destructive to animal life. 



Let us analyze this a little further. What do we mean by the 

 statement that leaf action is keeping our atmosphere respirable 

 today ? 



One of the great drawbacks or hindrances to rapid investigation 

 in an}' of the natural sciences is the temptation to depart from the 

 main line of study, and to search out tlie side questions ; because 

 nature is so complex in her structure that we cannot search one 

 line far without finding it linked and interlinked with those which 

 seem to be of equal, or even greater, importance. Keeping this 



