Marietta's second hypothesis more specially concerns the
chemical nature of plants ; he supposes that several of his
principes grossiers are contained in every plant, and he endeavours first to explain their source ; the motes in the air, he
says, which when burnt by lightning smell of sulphur, are
carried by rain into the earth, and parts of them are taken up
into the plant. Moreover distillation in all plants produces a
water, which the chemists call phlegma, and also acids and
ammonia, and if the residuum is burnt there remains an ash,
from which we obtain an earth which is without taste and
insoluble in water, and fixed salts ; these salts differ from one
another according as they are mixed with more or less acid and
ammoniacal spirit or other unknown principles, which the fire
could not volatilise. It is not to be wondered at that these
principles are found in plants, since they derive their food from
the earth which contains them. We see how great has been the advance since the time when Van Helmont believed that he had proved by his experiment, that all the materials in plants
come from pure water.
It remained to confront one view of the source of the substances in plants, which was also drawn from the treasure-house of Aristotelian conceptions, and was still in vogue. It was supposed that the very materials of which the plant is composed were contained in their own form in the earth, and had only to be taken up by the roots. Aristotle had himself said: 'Everything feeds on that of which it consists, and everything feeds on more than one thing; whatever appears to feed only on one thing, as the plant on water, feeds on more than one thing, for earth in the case of the plant is mixed with the water; therefore the country-people water plants with mixtures of things.' This passage might leave some doubt about Aristotle's view, if we did not find the following: 'As many savours as there are in the rinds of fruits, so many it is plain prevail also in the earth. Therefore also many of the old philosophers said, that the water is of as many kinds us the ground through
