THE FOOD OF PLANTS MINERAL. 287 



the purely mineral state. So fond are some animals of salt, 

 that in the vast prairies of America the herds are prevented 

 from straying by establishing salt-washes, or saline drinking- 

 places, within certain bounds. It must be confessed, then, that 

 salt is a real exception to the absoluteness of the rule referred 

 to that it is essentially an aliment that it is decomposed or 

 chemically altered within the body and that it is, almost 

 always in man, derived directly from the mineral kingdom. 

 It may be worth while to notice that iron is sometimes 

 required by the human body to a greater extent than the diet 

 at the time can afford, and that the direct use of iron in the 

 mineral state contributes to the restoration of health. Here, 

 doubtless, it is a medicine yet it is a medicine which 

 operates its effect by acting as an aliment. The disease in 

 which this exception is most remarkably observed is chlorosis 

 one of those maladies in which, in ruder times, the patient 

 was often detected eating earth, ashes, and the like, as if a 

 natural instinct suggested a search after the mineral food 

 required at the time by the system. It would be curious to 

 inquire if animals in the state of freedom ever frequent chalyb- 

 eate springs. 



But it is almost trifling to dwell on such exceptions as 

 these. A more important, though almost purely speculative 

 difficulty, is involved in one of the statements made above ; 

 namely, that the food of plants is exclusively mineral. This, 

 however, seems to be the view which now stands nearly 

 established. The food of plants consists of water, carbonic 

 acid, ammonia, and certain earthy and saline matters. A 

 great part of the carbonic acid which serves for food to plants 

 is derived from the atmosphere, and this portion of their food 

 is beyond doubt in the mineral state. But plants also take in 

 carbonic acid from the soil, set free during the decomposition 

 of organic matters therein contained ; and the question which 



