A HISTORY OF SCIENCE 



(Essays Literary and Philosophical: Edinburgh), have 

 the appearance of globules of quicksilver. And hence 

 leaves with the upper surfaces on water wither as soon 

 as in the dry air, but continue green for many days if 

 placed with the under surface on water, as appears 

 in the experiments of Monsieur Bonnet (Usage des 

 Feuilles). Hence some aquatic plants, as the water- 

 lily (Nymphfza), have the lower sides floating on the 

 water, while the upper surfaces remain dry in the air. 



"5. As those insects which have many spiracula, or 

 breathing apertures, as wasps and flies, are immediately 

 suffocated by pouring oil upon them, I carefully covered 

 with oil the surfaces of several leaves of phlomis, of 

 Portugal laurel, and balsams, and though it would not 

 regularly adhere, I found them all die in a day or two. 



"It must be added that many leaves are furnished 

 with muscles about their foot-stalks, to turn their 

 surfaces to the air or light, as mimosa or Hedysarum 

 gyrans. From all these analogies I think there can be 

 no doubt but that leaves of trees are their lungs, giving 

 out a phlogistic material to the atmosphere, and ab- 

 sorbing oxygen, or vital air. 



"6. The great use of light to vegetation would ap- 

 pear from this theory to be by disengaging vital air 

 from the water which they perspire, and thence to facil- 

 itate its union with their blood exposed beneath the 

 thin surface of their leaves ; since when pure air is thus 

 applied it is probable that it can be more readily ab- 

 sorbed. Hence, in the curious experiments of Dr. 

 Priestley and Mr. Ingenhouz, some plants purified less 

 air than others that is, they perspired less in the sun- 

 shine ; and Mr. Scheele found that by putting peas into 



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