This cycle of changes can be repeated as often as we please; the quantities of hydrogen and oxygen which are obtained when we choose to stop the process are exactly the same as the quantities of those substances which disappeared in the first operation whereby water was produced.
Hence, water is an intimate union of hydrogen and oxygen; and, in this sense, water may be said to contain hydrogen and oxygen.
The alchemist would have said, the properties of hydrogen and oxygen are destroyed when these things unite to form water, but the essence, or substratum, of each remains.

The chemist says, you cannot discover all the properties of hydrogen and oxygen by examining these substances apart from one another, for one of the most important properties of either is manifested only when the two mutually react: the formation of water is not the destruction of the properties of hydrogen and oxygen and the revelation of their essential substrata, it is rather the manifestation of a property of each which cannot be discovered except by causing the union of both.
There was, then, a certain degree of accuracy in the alchemical description of the processes we now call chemical changes, as being the removal of the outer properties of the things which react, and the manifestation of their essential substance.

But there is a vast difference between this description and the chemical presentment of these processes as reactions between definite and measurable quantities of elements, or compounds, or both, resulting in the re-distribution, of the elements, or the separation of the compounds into their elements, and the formation of new compounds by the re-combination of these elements.
Let us contrast the two descriptions somewhat more fully.
The alchemist wished to effect the transmutation of one substance into another; he despaired of the possibility of separating the Elements whereof the substance might be formed, but he thought he could manipulate what he called the _virtues_ of the Elements by a judicious use of some or all of the three Principles, which he named Sulphur, Salt, and Mercury.

He could not state in definite language what he meant by these Principles; they were states, conditions, or qualities, of classes of substances, which could not be defined.

The directions the alchemist was able to give to those who sought to effect the change of one thing into another were these.