230 
HISTORY OF BOTAKY. 
aided bj tlie discoveries made in pneumatic chemistry of tlio 
existence of oxygen, hydrogen, and carbonic acid gases, formed 
a new era in the history of vegetable physiology. It was proved 
that vegetables do ultimately consist of oxygen^ hydrogen^ and 
carbon^ 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 water, 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 nature-list whose labors in point of utility will best 
bear a comparisor) with those of Linnaeus, 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 natural 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 hi 
the seed,, which were found to be universal ; this was perfected 
and published by his nephew, Antoine- Laurent de Jussieu^ and 
is now generally received as the best mode of natural classifica- 
tion which has yet been discovered. This method is called nat- 
ural because it aims to bring into groups such genera of plants 
as resemble each other in medicinal and other properties^ while 
the system of Linngeus 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 Linnsean 
classes the poisonous fiag 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 similar contrasts in the qualities of the plants 
which are collected into the same classes. Kor 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 ol 
Botany, but is the hey to the natural method^ by which^ alone, we 
358. Character of Jussieu— His natural method of classing plants. 
