230 HISTOET OF BOTANY. 



aided by the discoveries made in pneumatic chemistry of the 

 existence of oxygen, hydrogen, and carbonic acid gases, formed 

 a new era in the history of vegetable physiology. It was ^proved 

 that vegetahles do ulthnately consist of oxygen^ hydrogen^ and 

 carton^ sometimes of a small quantity of nitrogen combined 

 with mineral salts, and often some silex, sulphur, and iron. 

 These elementary substances were found to be diffused through 

 air and watei', and the animal and vegetable substances which 

 the latter holds in solution : the green parts of vegetables were 

 observed to exhale oxygen in the light, and carbonic acid gas 

 in the dark ; and the carbon left by the decomposition of the 

 carbonic acid, was shown to be incorporated into the vegetable 

 substance giving to the wood its strength and hardness. 



358. The naturalist whose labors in point of utility will best 

 bear a comparison with those of Linnseus, is Bernard de Jus- 

 sieu. An unambitious man, he was remarkable for the extent 

 of his knowledge, the penetration of his genius, and the solidity 

 of his judgment. The love of truth and science were with him 

 sufficient incitements to the most severe labor. " Many of our 

 contemporaries," says Mirbel, " knew this sage ; they say that 

 never have they seen so much knowledge combined with so 

 high a degree of candor and modesty." To this botanist we are 

 indebted for a natiu-al method of classification superior to those 

 of his predecessors, and one on which has been founded the 

 system of natural classification now in use. Jussieu proposed 

 a method of classing plants according to certain distinctions in 

 the seed^ which were found to be universal ; this was perfected 

 and j)ublished by his nephew, Antoine-Lanrent de Jussien^ and 

 is now generally received as the best mode of natural classifica- 

 tion which has yet been discovered. This metliod is called nat- 

 ural hecause it aims to hring into groiqjs such genera of jplants 

 as resemble each other in medicinal and other jproperties^ while 

 the system of Linnseus is called artificial because by a certain 

 rule plants which have no such resemblance in their properties 

 are brought together. We therefore find in one of the Linn£ean 

 classes the poisonous flag and the nutritious grass, the grain 

 which supports life and the darnel which destroys it ; in an- 

 other the healthful potato and the poison mandrake, the deadly 

 hemlock and the grateful coriander. Throughout this system 

 we meet with sinular contrasts in the qualities of the plants 

 which are collected into the same classes. JTor are their external 

 appearances less unlike ; for here the oleander and pig-weed, 

 the tulip and the dock, meet in the same classes. This system,. 

 it should always be lemembered, is not the whole science of 

 Botany, but is the hey to the natural method^ hy which^ alone, we 



358. Character of Jussieu — His natural method of classing plants. 



