the other principle of combustible things was scarcely a Principle in the alchemical sense, for, in the case of metals at any rate, it remained when the things which had contained it were burnt, and could be seen, handled, and weighed.  To say that metals are composed of phlogiston and an earthy substance, was to express facts in such a language that the expression might be made the basis of experimental inquiry; it was very different from the assertion that metals are produced by the spiritual actions of the three Principles, salt, mercury and sulphur, the first of which is not salt, the second is not mercury, and the third is not sulphur.  The followers of Stahl often spoke of metals as composed of phlogiston and an element of an earthy character; this expression also was an advance, from the hazy notion of Element in purely alchemical writings, towards accuracy and fulness of description.  An element was now something which could he seen and experimented with; it was no longer a semi-spiritual existence which could not be grasped by the senses.

The phlogistic theory regarded the calcination of a metal as the separation of it into two things, unlike the metal, and unlike each other; one of these things was phlogiston, the other was an earth-like residue.  The theory thought of the re-formation of a metal from its calx, that is, the earthy substance which remains after combustion, as the combination of two things to produce one, apparently homogeneous, substance.  Metals appeared to the phlogisteans, as they appeared to the alchemists, to be composite substances.  Processes of burning were regarded by alchemists and phlogisteans alike, as processes of simplification.

The fact had been noticed and recorded, during the middle ages, that the earth-like matter which remains when a metal is calcined is heavier than the metal itself.  From this fact, modern investigators of natural phenomena would draw the conclusion, that calcination of a metal is an addition of something to the metal, not a separation of the metal into different things.  It seems impossible to us that a substance should be separated into portions, and one of these parts should weigh as much as, or more than, the whole.

The exact investigation of material changes called chemistry rests on the statement that mass, and mass is practically measured by weight, is the one property of what we call matter, the determination whereof enables us to decide whether a change is a combination, or coalescence, of different things, or a separation of one thing into parts.  That any part of a material system can be removed without the weight of the portion which remains being less than the original weight of the whole system, is unthinkable, in the present state of our knowledge of material changes.