272 WATER IS FOOD. [BOOK n. 



" While, however, all experiments combine to prove that 

 carbonic acid is the most essential of the elements upon which 

 plants are nourished, it is necessary that the student should be 

 aware that other species of matter are constantly taken into the 

 system, and probably, therefore, contribute to their nutrition. 



" Water is one of these. Although we know that a very 

 large proportion of all the water absorbed by a plant is lost 

 again by evaporation, yet the experiments of Theodore de 

 Saussure have shown that a portion of it is actually solidified. 

 He found that when plants are grown in a close vessel, in an 

 artificial atmosphere, containing a little carbonic acid, the 

 weight which the plant acquired in a given time was aug- 

 mented, 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 Peri- 

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

 1-1 grain from water, acquired 5-^, when they were at the 

 same time able to procure carbon. The same excellent ob- 

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

 all the weight which a plant can gain, by fixing carbon, by 

 depositing earthy, saline, alkaline, metallic, and other matters 

 which it borrows from the soil, we shall not be able to account 

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

 plant. The other nineteen-twentieths must, therefore, be 

 fixed water. Whatever errors there may be in calculations of 

 this nature, there cannot be much 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 water 

 of crystallisation independently of their water of vegetation." 



as there is no evidence of ammonia being thrown forth from the bowels of the 

 earth at all times in quantity proportioned to the waste of it necessarily sustained 

 at the surface by decomposition, as into uncombined hydrogen and nitrogen, that 

 Liebig's view of ammonia infers the same limitation of the existence of the 

 organic kingdoms to a few thousand years, as is deduced from the hypothesis of 

 organic matter being the food of plants. Here, therefore, he dissents from 

 Liebig, contending that ammonia must be produced from the nitrogen of the 

 atmosphere, and showing the probability of what is taught by Professor Johnson, 

 namely, that the nitrogen of nitrates, formed from the atmosphere, is fixed by 

 plants, as well as the nitrogen of ammonia. Annals of Natural Hi-story, xv. 350. 



