Of the seven principles, Vibration is the one that most plainly borrows the language of the science of its own moment. By 1908 the wave theory of light was settled, the electromagnetic spectrum was a schoolroom fact, and wireless telegraphy had crossed the Atlantic. The Kybalion takes that vocabulary and carries it past where the physics goes, holding that thought, emotion and spirit differ from matter only in the rate at which they vibrate.
The move was not the book's invention. William Walker Atkinson had published Thought Vibration, or the Law of Attraction in the Thought World in 1906, arguing that mental states radiate and draw like states from others. Reading the two books side by side is among the strongest reasons to think one hand wrote both.
What follows in practice is the doctrine of mental transmutation. If states differ only by rate, then changing a state is a matter of altering rate rather than defeating an opposite, and the later chapters build their whole method on that. It is worth separating the method from the analogy that dresses it. The analogy is nineteenth century and does not survive contact with modern physics, which has no scale on which grief and granite are the same substance at different speeds. The practice it recommends, deliberate attention to shifting an emotional state instead of fighting it, is not answerable to that failure and did not depend on it.
Sources The Kybalion: The Definitive EditionPhilip Deslippe (ed.) with Thought Vibration, or the Law of Attraction in the Thought WorldWilliam Walker Atkinson