But it is also to be remembered that the alchemists were acquainted with, and practised, many processes which we should now describe as operations of manufacturing and technical chemistry; and the practical usefulness of these processes bore testimony, of the kind which convinces the plain man, to the justness of their theories.
I have always regarded two facts as most interesting and instructive: that the doctrine of the essential unity of all things, and the simplicity of natural order, was accepted for centuries by many, I think one may say, by most men, as undoubtedly a true presentation of the divine scheme of things; and, secondly, that in more recent times people were quite as certain of the necessary truth of the doctrine, the exact opposite of the alchemical, that the Creator had divided his creation into portions each of which was independent of all the others.

Both of these schemes were formed by the same method, by introspection preceding observation; both were overthrown by the same method, by observation and experiment proceeding hand in hand with reasoning.

In each case, the humility of science vanquished the conceit of ignorance.
The change from alchemy to chemistry is an admirable example of the change from a theory formed by looking inwards, and then projected on to external facts, to a theory formed by studying facts, and then thinking about them.

This change proceeded slowly; it is not possible to name a time when it may be said, here alchemy finishes and chemistry begins.

To adapt a saying of one of the alchemists, quoted in a former chapter; alchemy would not easily give up its nature, and fought for its life; but an agent was found strong enough to overcome and kill it, and then that agent also had the power to change the lifeless remains into a new and pure body.