Lucien Poincaré, writing in 1909 as a French inspector-general of public instruction, set out to clarify what he called a 'confusion more apparent than real' in physics. His book, translated into English as The New Physics and Its Evolution, does not merely list discoveries but attempts to show how recent experiments—especially those involving gaseous conduction and electrolytic ions—were forcing physicists to reconsider the very foundations of their science. Poincaré's tone is measured, his method analytical: he relies on 'only the' evidence available, avoiding the partisan controversies that surrounded radium and atomic speculation.

The Electric Atom Emerges from Electrolysis

Poincaré devotes careful attention to the laws of Faraday, using them to build a bridge between chemistry and electricity. He notes that the first law—linking the quantity of electricity to the mass deposited on electrodes—implies that all ions in a given solution carry equal charges. The second law, which ties charge to valency, leads him to quote Helmholtz's striking conclusion: electricity itself must be 'composed of elementary parts which behave like atoms of electricity.' This is not presented as a finished theory but as a logical inference from experimental data. Poincaré then calculates the charge of a hydrogen ion, arriving at a figure of 1.3 × 10-20 electromagnetic units, and compares the slow drift of ions in liquids (0.3 mm per second under one volt per centimetre) with the far higher speeds expected in gases. The contrast sets up a key tension: the same fundamental particles behave very differently depending on the medium.

The Ether as a Receptacle of Energy

In his chapter on the luminiferous ether, Poincaré traces the concept from Descartes through Newton's eclipse and Fresnel's revival. He describes the ether as a 'subtle matter which is the receptacle of the energy of the universe,' a phrase that captures both its ambition and its vagueness. The author does not endorse the ether uncritically; instead, he presents it as a hypothesis that has been 'marvellously fitted' to predict optical phenomena, yet one that faces new challenges from electromagnetic theory. The discussion is notably restrained: Poincaré acknowledges the ether's explanatory power while hinting at the strains that Maxwell's equations and later experiments would place on it. This section exemplifies his method of laying out a framework, testing it against recent work, and leaving the reader to weigh the evidence.

Gaseous Conduction and Moving Centres

A recurring pattern in Poincaré's account is the shift from static models to dynamic ones. He observes that discoveries about electrically conducting gases 'almost force upon us' the idea of 'electrified centres moving through the field.' This phrasing—'almost force'—reveals his cautious empiricism: the evidence is compelling but not yet conclusive. He links this to the older theory of electrolytic conductivity, suggesting that the same ionic mechanism may operate in gases, only with far greater speeds. The reader is left to see how a single conceptual thread—charged particles in motion—runs through seemingly disparate phenomena. Poincaré does not claim to have the final answer; instead, he highlights the direction in which physics is moving, from static forces to mobile, particulate agents.

Poincaré's book is best read as a snapshot of a discipline in transition, not as a definitive statement. Readers should attend to his repeated use of phrases like 'it will be useful' and 'we shall see,' which signal that he is guiding them through an ongoing argument rather than delivering settled truths. The value of the work lies less in its conclusions than in its demonstration of how experimental evidence gradually reshapes theoretical commitments—a process that, as Poincaré shows, is as much about clarifying questions as about finding answers.