THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 433 



PART FIRST. 

 GENERAL HISTORY, AND STATEMENT OF THE QUESTION. 



Section I.— INTRODUCTION, AND EARLY HISTORY. 

 The facts at the present time generally accepted regarding the ultimate composition, 

 and the sources of the constituents, of plants, have, for the most part, received their 

 preponderating weight of proof within the limits of the present century. But it is to 

 the century preceding it that we must look for the establishment of much that was 

 essential as the foundation of those advances which have since been made. 



Whatever may be the value at present attached to the particular views of Hales 

 regarding the composition and the sources of vegetable matter, we must accord to his 

 labours, in the early part of the eighteenth century, the merit of having been guided by 

 a proper spirit of experimental inquiry. Nor did he fail in applying to good account, 

 and even in extending, the then existing knowledge of the material things around him 

 which were apparently involved in the mysterious processes of vegetable growth. 



With our present knowledge, however, of the general composition of plants, and of 

 the sources of their constituents, it is easy to see how essential was a proper under- 

 standing of the chemistiy of the air, and of water, to any true conceptions of the mate- 

 rial changes involved in the vegetative process. It can, indeed, hardly excite surprise, 

 that what may be called the germs of our present knowledge of the chemistiy of plant- 

 growth came forth almost simultaneously with the now adopted views of the compo- 

 sition of those universal, though not exclusive, media of vegetation — air, and water. 



Accordingly, it is between the [dates of 1770 and 1800 that we find Black, Scheele, 

 Priestley, Lavoisier, Cavendish, and Watt establishing for us the facts that common 

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

 acid itself is composed of carbon and oxygen ; and that water is composed of hydrogen 

 and oxygen ; and it is within the same period that Priestley and Ixgexhousz, Senne- 

 biee and Woodhouse, laboured to show the mutual relations of these bodies and vege- 

 table growth. 



But the observers last mentioned seem to have had more prominently in view the 

 question of the influence of plants upon the media with which they were surrounded, 

 than that of the influence of these media in contributing materially to the increased 

 substance of plants themselves. Following closely on their footsteps, both in point of 

 time and in general plan of research, came De Saussure. His labours were conducted 

 towards the end of the last century and in the beginning of the present one ; and their 

 results, and the arguments he founded upon them, published by him in 1804, may be 

 said to have indicated, if not indeed established, many of the most important facts with 

 which we are yet acquainted regarding the sources of the constituents stored up by the 

 growing plant. To De Saussure we owe the experimental, and even quantitative, illus- 

 tration of the fact, that plants in sunlight increase in their amounts of carbon, hydro- 

 gen, and oxygen, at the expense of carbonic acid and of water. It is remarkable, too, 

 that, in the case of the "main experiment he cites on the point, he, with his very imper- 



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