XXIV INTRODUCTION. 



ganic, or earthy and saline matters. The presence 

 of Nitrogen was formerly greatly overlooked, in 

 analyses of vegetable substances ; it is contained in 

 less quantity than the other three elements of 

 organic matter, and was very commonly regarded as 

 being merely accidental, and not a necessary consti- 

 tuent of plants. Improved modes of analysis have 

 proved that nitrogen always exists in the same pro- 

 portion, in certain constituents of plants; -and, as it 

 appears that these substances are also those which 

 form the most valuable part of food, it becomes a 

 question of the first importance — Whence do the 

 plants derive their Nitrogen? They obtain it, prin- 

 cipally, if not wholly, from the air; they do not 

 absorb it in the free and uncombined form, but- they 

 absorb it combined with Hydrogen and with Oxygen, 

 in the states of Ammonia and Nitric Acid. 'The 

 importance of the earthy substances in plants was, 

 likewise, greatly overlooked formerly. It has been 

 proved, by repeated experiments, that these sub- 

 stances are of the greatest importance in the growth 

 of plants, being quite essential to their development. 

 Although much has been done, and although che- 

 mists have labored to remove the perplexities which 

 encompassed the subject, there is still a very great 

 deal which requires investigation; many important 



