Phlogiston

Phlogiston (from the Ancient Greek φλογιστόν phlogistón "burning up") was a fire-like element contained within combustible bodies and released during combustion. The air was thought to have a limited capacity to absorb the phlogiston released, this being the reason why combustion did not take place for long in an enclosed container.

The theory was first postulated in 1667 by Johann Joachim Becher (reformed in 1703 by Georg Ernst Stahl) as an attempt to explain processes of burning such as combustion, metabolism, and the rusting of metals, which are now collectively known as oxidation.

During the eighteenth century, as it became clear that metals gained weight when they burned or were oxidized, even though they were supposed to have lost phlogiston. The threory remained dominant until Antoine-Laurent Lavoisier (1743 – 1794) showed that combustion requires a gas that has weight (oxygen) and could be measured by means of weighing closed vessels.

Theosophical use

In one of his letters, Mahatma K.H. says:

Well, we believe in the much laughed at phlogiston (see article “What is force and what is matter?” Theosophist, September), and in what some natural philosophers would call nisus, the incessant though perfectly imperceptible (to the ordinary senses) motion or efforts one body is making on another — the pulsations of inert matter — its life. The bodies of the Planetary spirits are formed of that which Priestley and others called Phlogiston and for which we have another name — this essence in its highest seventh state forming that matter of which the organisms of the highest and purest Dhyans are composed, and in its lowest or densest form (so impalpable yet that science calls it energy and force) serving as a cover to the Planetaries of the 1st or lowest degree.

In the article referred to above Mme. Blavatsky was discussing the nature of force and matter. In it, she argued that forces such as electricity were a form of matter (this, before the electron was discovered), and relate them to the idea of phlogiston, although not in the sense of some kind of matter with weight:

. . .it is not in the least unscientific to speak of the substantiality of the so-called Forces. Subject to some future specific name, this force is substance of some kind, and can be nothing else; and perhaps one day Science will be the first to re-adopt the derided name of phlogiston.

The Phlogiston of Stahl [was] a theory of combustion taught by Aristotle and the Greek philosophers. . . . Lavoisier, as it is well known, did not add any new fact of prime importance by upsetting the phlogiston theory, but only added “a grand generalization.” But the Occultists prefer to hold to the fundamental theories of ancient sciences. No more than the authors of the old theory, do they attach to phlogiston—which has its specific name as one of the attributes of Akasha—the idea of weight which the uninitiated generally associate with all matter. And though to us it is a principle, a well-defined essence, whereas to Stahl and others it was an undefined essence—yet, no more than we, did they view it as matter in the sense it has for the present men of science. As one of their modern professors puts it: “Translate the phlogiston by energy, and in Stahl’s work on Chemistry and Physics, of 1731, put energy where he wrote phlogiston, and you have . . . our great modern doctrine of conservation of energy.” Verily so; it is the “great modern doctrine,” only—plus something else, let me add. Hardly a year after these words had been pronounced, the discovery by Professor Crookes of radiant matter—of which, further on—has nigh upset again all their previous theories.

The "radiant matter" was what today is known as "plasma", the fourth state of matter. By heating a gas its molecules or atoms are ionized (reducing or increasing the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions.