232 HISTORT OF BOTANY. 
The observations of Priestley, Saussure, and others, aided by the 
discoveries made in pneumatic chemistry, of the existence of oxy- 
gen, liydrogen, and carbonic acid gases, formed a new era in the 
history of vegetable physiology. It was proved that vegetables do 
ultimately consist of oxygen^ liydrogen, 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 vege- 
table substances which the latter holds in solution : the green parts 
of vegetables were observed to exhale oxygen in the hght,.and car- 
bonic acid gas in the darkj and the carbon left by the decomposi- 
tion of the carbonic acid, was shown to be incorporated into the vege- 
table substance, giving to the wood its strength and hardness. 
The naturahst whose labours, in point of utihty, will best bear a 
comparison with those of Linnaeus, is Bernard de Jussieu. He 
w^as remarkable for the extent of his knowledge, the penetration 
of his genius, and the sohdity of his judgmejit. He is said to have 
been unambitious. The love of truth and science were with him 
sufficient excitements to the most severe labour. " 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 candour and modesty." To this botanist we are indebted 
for a natural method of classification, superior to those of his pre- 
decessors. 
Jussieu proposed a method of classing plants according to c.er- 
tain distinctions in 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 tlie best mode of natiu^al 
classification v/hich has yet been discovered. This method is called 
natural, because it aims to bring into groups such genera of plants 
as resemble each other in medicinal and other properties ; while the 
system of Linnasus is c.dled 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 Linnaean classes 
the poisonous flag and the nutritious grass, the grain which supports 
life and the darnel which destroys it; in another, the healthful pota- 
to 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. Nor are their external appearances less unlike ; for here 
the oleander and pigweed, the tuhp and the dock, meet in the same 
classes. This s^^stem, it should always be remembered, is not the 
whole science of Botany, but is the key to the natural method, by 
which alone, we should find great difliculty in ascertaining the 
names of plants; it is, as it were, a stepping-stone by which we must 
ascend to the valuable knowledge which cannot weh be reached in 
any other way. The more practical a botanist becomes, the less 
need he has for this assistance ; the eye becomes quick to seize on 
natural characters without reference to the dictionary, as the arti- 
ficial system is aptly termed. Thus a pupil, in studying a language, 
may, in time, be able to dispense with his dictionary; though he 
could not have proceeded, at first, without its assistance. For more 
particular explanations of Jussieu's method, you are referred to the 
comparison of that with the method of Linnaeus and Tournefort in 
the remarks on classification, and to the Natural Orders contained in 
the appendix. 
Priestley, &c.— Ciiaracter of Jussieu— Natural method of classing plants. 
