CHAP. IX. FOOD OF PLANTS. 307 



merited, not only by the quantity of carbon produced by the 

 decomposition of carbonic acid, but to a much more consider- 

 able extent, which could only be ascribed to its having 

 fixed a considerable quantity of water; thus plants of the 

 Periwinkle, which, in a vessel without carbonic acid, had gained 

 If grain from water, acquired 5t^o) when they were at the same 

 time able to procure carbon. The same excellent observer 

 has computed, that if we calculate with the utmost care all 

 the weight which a plant can gain, either by fixing carbon, or 

 by depositing earthy, saline, alkaline, and metallic matter 

 which it borrows from the soil, or by respiring oxygen, or 

 from the soluble matter of soil, we shall not be able to account 

 for more than a twentieth part of the real weight of such a 

 plant. The other nine teen-twentieths must, therefore, be 

 fixed water. Whatever errors thei-e may be in calculations 

 of this nature, there cannot be a doubt that they are correct 

 to so considerable an extent as to oblige us to admit that 

 water forms a considerable part of the solid tissue of plants ; 

 so that it would appear that, like minerals, plants have a wa- 

 ter of crystallization independently of their water of vegetation. 



" As it has been pretty well made out that all the oxygen 

 given off by plants is produced by the decomposition of car- 

 bonic acid, and as no one has ever been able to detect the 

 emission of hydrogen by any plants except Mushrooms, it is 

 inferred that, if the water which is consumed by plants is ever 

 decomposed, it is in the formation of the various secretions 

 which contain more oxygen (acids), or more hydrogen (oils), 

 than water ; but as the greater part of vegetable substances, 

 such as gum, sugar, fecula, &c., contain oxygen and hydrogen 

 in the same proportions as water, it can hardly be doubted 

 that the greater part is undecomposed and simply fixed. 



" It was formerly thought that nitrogen, or azote, has no- 

 thing to do with the nutrition of plants, and that in those cases 

 where it was met with it was merely in a state of separation 

 from the atmospheric air which had been inhaled and depriv- 

 ed of its oxygen and carbonic acid. But its constant presence 

 in combination with the tissue of Mushrooms and of Crucife- 

 rous plants, in gluten, and what chemists call vegetable albu- 

 men, and also in vegetable alkalies, seems a sufficiently strong 



X 2 



