IV PREFACE. 



from chemistry, which teaches the mode of investigating the compostion and of 

 studying the characters of the different substances from which plants derive their 

 nourishment. 



The chemical forces play a part in all the processes of the living animal organ- 

 ism ; and a number of transformations and changes in the living body are exclu- 

 sively dependent on their influence. The diseases incident to the period of growth 

 of man, contagion and contagious matters, have their analogues in many chemical 

 processes. The investigation of the chemical connection subsisting between those 

 actions proceeding in the living body, and the transformations presented by chemical 

 compounds, has also been a subject of my inquiries. A perfect exhaustion of this 

 subject, so highly important to medicine, cannot be expected without the co-opera- 

 tion of physiologists. Hence I have merely brought forward the purely chemical 

 part of the inquiry, and hope to attract attention to the subject. 



Since the time of the immortal author of the "Agricultural Chemistry," no 

 chemist has occupied himself in studying the applications of chemical principles to 

 the growth of vegetables, and to organic processes. I have endeavoured to follow 

 the path marked out by Sir Humphry Davy, who based his conclusions only on 

 that which was capable of inquiry and proof. This is the path of true philoso- 

 phical inquiry, which promises to lead us to truth the proper object of our 

 research. 



In presenting this report to the British Association I feel myself bound to convey 

 my sincere thanks to Dr. Lyon Plairfair, of St. Andrew's, for the active assistance 

 which has been afforded me in its preparation by that intelligent young chemist, 

 during his residence in Giessen. I cannot suppress the wish that he may succeed 

 in being as useful, by his profound and well grounded knowledge of chemistry, 

 as his talents promise. 



JUSTUS LIEBIG. 



Gixsen, September 1, 1852. 



