12 TRA^NSACTIONS. 



nitrogen and hydrogen, sometimes exists in small quantity in 

 the air, and is produced still more abundantly by fermenting 

 manures. Nitric acid likewise is sometimes found in minute 

 quantity in the atmosphere, and its absortion would furnish 

 nitrogen as well as oxygen. 



As to the inorganic matter of plants, the sulphur, phosphorus, 

 lime, silex, iron, manganese, &c., it must nearly all be derived 

 from the soil, since water alone can hold it in solution. Chlo- 

 rine perhaps may, as Dr. Dana suggests, be derived from the 

 atmosphere in the condition of common salt, which he has 

 shown to exist probably in tiie air. 



Such are the ordinary sources whence the food of the vege- 

 tible world is derived. But there is one principle of vegetable 

 physiology of great importance and deep philosophical interest, 

 that should be noticed, since its neglect has been a fruitful 

 source of controversy among scientific men. It cannot be 

 doubted that plants like animals have the power of adapting 

 themselves to circumstances ; so that, if they cannot obtain 

 nourishment from one source, they are able to derive it from 

 another. It is well known that some plants will flourish in 

 pure water, others suspended in the air ; and on some marly 

 soils, destitute of vegetable matter, abundant crops may be 

 raised without manure through an indefinite number of years ; 

 though in all these cases, probably, plants are less robust and 

 prolific than if supplied with food both from the soil and the 

 air. But their ability to draw a less perfect subsistence from 

 different sources is another interesting evidence of Divine fore- 

 sight and benevolence. 



Chemists have not been satisfied with ascertaining the na- 

 ture and origin of the nutriment of vegetable nature. They 

 have attempted to foUoAV the crude materials through their va- 

 rious and most delicate metamorphoses, till they become con- 

 verted into the different remarkable compounds which plants 

 produce. And, though much of the chemistry of these changes 

 is concealed, yet we can see what are the most important 

 agents concerned ; and heat, light, and electricity, stand at the 

 head of the4ist. It can hardly be doubted that the rootlets of 



