The great fault of the theory of phlogiston, considered as a general conception which brings many facts into one point of view, and leads the way to new and exact knowledge, was its looseness, its flexibility.  It was very easy to make use of the theory in a broad and general way; by stretching it here, and modifying it there, it seemed to cover all the facts concerning combustion and calcination which were discovered during two generations after the publication of Stahl’s books.  But many of the subsidiary hypotheses which were required to make the theory cover the new facts were contradictory, or at any rate seemed to be contradictory, of the primary assumptions of the theory.  The addition of this ancillary machinery burdened the mechanism of the theory, threw it out of order, and finally made it unworkable.  The phlogistic theory was destroyed by its own cumbersomeness.

A scientific theory never lasts long if its fundamental assumptions are stated so loosely that they may be easily modified, expanded, contracted, and adjusted to meet the requirements of newly discovered facts.  It is true that the theories which have been of the greatest service in science, as summaries of the relations between established facts, and suggestions of lines of investigation, have been stated in terms whose full meaning has gradually unfolded itself.  But the foundations of these theories have been at once so rigidly defined and clearly stated as to be incapable of essential modification, and so full of meaning and widely applicable as to cover large classes of facts which were unknown when the theories were constructed.  Of the founders of the lasting and expansible theories of natural science, it may be said, that “thoughts beyond their thoughts to those high bards were given.”

CHAPTER XI.

THE EXAMINATION OF THE PHENOMENA OF COMBUSTION.

The alchemists thought that the most effectual method of separating a complex substance into more simple substances was to subject it to the action of heat.  They were constantly distilling, incinerating, subliming, heating, in order that the spirit, or inner kernel of things, might be obtained.  They took for granted that the action of fire was to simplify, and that simplification proceeded whatever might be the nature of the substance which was subjected to this action.  Boyle insisted that the effect of heating one substance may be, and often is, essentially different from the effect of heating another substance; and that the behaviour of the same substance when heated, sometimes varies when the conditions are changed.  He takes the example of heating sulphur or brimstone:  “Exposed to a moderate fire in subliming pots, it rises all into dry, and almost tasteless, flowers; whereas being exposed to a naked fire, it affords store of a saline and fretting liquor.”  Boyle thought that the action of fire was not necessarily to separate a thing into its principles or elements,