508 KEPORT— 1880. 



special views put forward in those -works, will — whether they be agricultural 

 chemists, vegetable physiologists, or animal physiologists — be the first to admit 

 how vast has been the stimulus, and how important has been the direction, given 

 to research in their own department, by the masterly review of then existing 

 knowledge, and the bold, and frequently sagacious, generalisations of one of the 

 most remarkable men of his time ! 



Confining attention to researches bearing upon agriculture, it will be well, 

 before attempting to indicate either the position established by Liebig's first works, 

 or the direction of the progress since made, to refer very briefly to the early history 

 of the subject. 



From what we now know of the composition and of the sources of the con- 

 stituents of plants, it is obvious that a knowledge of the composition of the atmo- 

 sphere and of water was essential to any ti-ue conception of the main features 

 of the vegetative process ; and it is of interest to observe that it was almost simul- 

 taneously with the establishment, towards the end of the last century, of definite 

 knowledge as to the composition of the air and of water, that their mutual rela- 

 tions with vegetation were first pointed out. To the collective labours of Black, 

 Scheele, Priestley, Lavoisier, Cavendish, and Watt, we owe the knowledge that 

 common air consists chiefly of nitrogen and oxygen, with a little carbonic acid ; 

 that carbonic acid is composed of carbon and oxygen ; and that water is composed 

 of hydrogen and oxygen ; whilst Priestley and Ingenhousz, Sennebier and Wood- 

 house, investigated the mutual relations of these bodies and vegetable growth. 

 Priestley observed that plants possessed the faculty of purifying air vitiated by 

 combustion or by the respiration of animals ; and, he having discovered oxygen, 

 it was found that the gaseous bubbles which Bonnet had shown to be emitted 

 from the surface of leaves plunged in water consisted principally of that gas. In- 

 genhousz demonstrated that the action of light was essential to the development of 

 these phenomena ; and Sennebier proved that the oxygen emitted resulted from 

 the decomposition of the carbonic acid taken up. 



So far, however, attention seems to have been directed more prominently to 

 the question of the influence of plants upon the media with which they were sur- 

 rounded, than to that of the influence of those media in contributing to the in- 

 creased substance of the plants themselves. Towards the end of the last century, 

 .and in the beginning of the present one, Tie Saussure followed up these inquiries ; 

 and in his work entitled, ' Recherches Chimiques snr la Vegetation,' published in 

 1804, he may be said to have indicated, if not indeed established, some of the most 

 important facts with which we are yet acquainted regarding the sources of the 

 constituents stored up by the growing plant. De Saussure illustrated experi- 

 mentally, and even to some extent quantitatively, the fact that in sun-light plants 

 increase in carbon, hydrogen, and oxygen, at the expense of carbonic acid and of 

 water ; and in the case of his main experiment on the point, he found the increase 

 in carbon, and in the elements of water, to be very closely in the proportion in 

 which these are known to exist in the carbohydrates. He further maintained the 

 «ssentialness of the mineral or ash constituents of plants ; he pointed out that they 

 must be derived from the soil ; and he called attention to the probability that the 

 incombustible constituents so derived by plants from the soil were the source of 

 those found in the animals fed upon them. 



With regard to the nitrogen which plants had already been shown to contain, 

 Priestley and Ingenhousz thought their experiments indicated that they absorbed 

 free nitrogen from the atmosphere ; but Sennebier and Woodhouse arrived at an 

 opposite conclusion. De Saussure, again, thought that his experiments showed 

 rather an evolution of nitrogen at the expense of the substance of the plant than 

 any assimilation of it from gaseous media. He further concluded that the source 

 of the nitrogen of plants was more probably the nitrogenous compounds in the soil, 

 and the small amoimt of ammonia which he demonstrated to exist in the atmo- 

 sphere. 



Upon the whole, De Saussure concluded that air and water contributed a 

 much larger proportion of the dry substance of plants than did the soils in which 

 they grew. In his view a fertile soil was one which yielded liberally to the plant 



