We may here introduce an illustration or two of this mode of procedure: 

Theories of gravitation and phlogiston.  Newton, suspecting that the influence of the earth’s attraction, gravity, may extend as far as the moon, and be the force that causes her to revolve in her orbit round the earth, calculated that, by her motion in her orbit, she was deflected from the tangent thirteen feet every minute; but, by ascertaining the space through which bodies would fall in one minute at the earth’s surface, and supposing it to be diminished in the ratio of the inverse square, it appeared that the attraction at the moon’s orbit would draw a body through more than fifteen feet.  He, therefore, for the time, considered his hypothesis as unsustained.  But it so happened that Picard shortly afterward executed more correctly a new measurement of a degree; this changed the estimated magnitude of the earth, and the distance of the moon, which was measured in earth-semidiameters.  Newton now renewed his computation, and, as I have related on a previous page, as it drew to a close, foreseeing that a coincidence was about to be established, was so much agitated that he was obliged to ask a friend to complete it.  The hypothesis was sustained.

A second instance will sufficiently illustrate the method under consideration.  It is presented by the chemical theory of phlogiston.  Stahl, the author of this theory, asserted that there is a principle of inflammability, to which he gave the name phlogiston, having the quality of uniting with substances.  Thus, when what we now term a metallic oxide was united to it, a metal was produced; and, if the phlogiston were withdrawn, the metal passed back into its earthy or oxidized state.  On this principle, then, the metals were compound bodies, earths combined with phlogiston.

Science and ecclesiasticism.  But during the eighteenth century the balance was introduced as an instrument of chemical research.  Now, if the phlogistic hypothesis be true, it would follow that a metal should be the heavier, its oxide the lighter body, for the former contains something—­phlogiston—­that has been added to the latter.  But, on weighing a portion of any metal, and also the oxide producible from it, the latter proves to be the heavier, and here the phlogistic hypothesis fails.  Still further, on continuing the investigation, it may be shown that the oxide or calx, as it used to be called, has become heavier by combining with one of the ingredients of the air.

To Lavoisier is usually attributed this test experiment; but the fact that the weight of a metal increases by calcination was established by earlier European experimenters, and, indeed, was well known to the Arabian chemists.  Lavoisier, however, was the first to recognize its great importance.  In his hands it produced a revolution in chemistry.

The abandonment of the phlogistic theory is an illustration of the readiness with which scientific hypotheses are surrendered, when found to be wanting in accordance with facts.  Authority and tradition pass for nothing.  Every thing is settled by an appeal to Nature.  It is assumed that the answers she gives to a practical interrogation will ever be true.