396 



NOTES AND ILLUSTRATIONS. 



the difference between night and day in this respect is extremely 

 curious. The substratum of ground on which beasts feed, as affected 

 b}'- caloric, is a subject which deserves greater chemical and physiologi- 

 cal investigation than has as yet been bestowed upon it. 



Note IV. Page 65. 



It has been doubted by some phytologists, whether trees generate 

 heat. I believe it is certain, notwithstanding what is cursorily stated 

 in the text, that frosts of very extraordinary severity will destroy trees. 

 The non-conducting property of wood may in some measure protect the 

 juices ; but their chemical composition, as here stated, is such that they 

 do not congeal, unless the cold be of the severest sort, and many degrees 

 below the freezing point of water. In w^eather so hard as to occasion 

 the juices to freeze, the wood, in the act of congelation, is violently 

 rent asunder ; but in the more common destruction of woody plants, 

 it is not so much the degree of cold that kills them, as the too 

 sudden re-application of heat. 



The ingenious Hassenfratz, to whom the chemical world is under 

 some obligations, held that vegetables are not fed by carbonic acid. 

 In a memoir on the nourishment of vegetables, read in 1792 to the 

 Royal Academy of Paris, having shown, as he conceived, that water 

 and air are insufficient for all the purposes of vegetation, he attemp- 

 ted in a second ingenious paper to prove that carbonic acid gas is 

 not decomposed and digested in the organs of growing vegetables, and 

 that they cannot be fed by it ; because oxygen, escaping from com- 

 bination in the decomposition of carbonic acid, and water escaping 

 in vapour in the state of gas, would absorb caloric, and produce 

 cold ; whereas, by the experiments of the late John Hunter, living 

 vegetables contain a degree of heat greater than that of the sur- 

 rounding atmosphere. The reason of this difference in opinion between 

 these two accurate inquirers may possibly be, that Hunter's experi- 

 ments were made only in the autumn, the winter, and early in the 

 spring, when the activity of vegetation was suspended, which does 

 not seem to have been the case respecting those of Hassenfratz. 



It appears, however, that both Ruchert and Senebier ascertained 

 that vegetables do decompose carbonic acid, retaining the carbon, and 

 emitting the oxygen. Dr Woodward made many experiments with 

 plants of mint growing in water, and found that a plant, in water 

 from the Thames, which must certainly have contained a large share 

 of carbonic acid, increased considerably more in weight, than a plant 

 growing in pure water. Schoppett, who examined the temperature 



