I have endeavoured throughout to rely only on the experiments in which we can place the most confidence, and, above all, to show how the ideas prevailing at the present day have been formed, by tracing their evolution, and rapidly examining the successive transformations which have brought them to their present condition. In order to understand the text, the reader will have no need to consult any treatise on physics, for I have throughout given the necessary definitions and set forth the fundamental facts. Moreover, while strictly employing exact expressions, I have avoided the use of mathematical language. Algebra is an admirable tongue, but there are many occasions where it can only be used with much discretion. Nothing would be easier than to point out many great omissions from this little volume; but some, at all events, are not involuntary. Certain questions which are still too confused have been put on one side, as have a few others which form an important collection for a special study to be possibly made later. Thus, as regards electrical phenomena, the relations between electricity and optics, as also the theories of ionization, the electronic hypothesis, etc., have been treated at some length; but it has not been thought necessary to dilate upon the modes of production and utilization of the current, upon the phenomena of magnetism, or upon all the applications which belong to the domain of Electrotechnics. L. POINCARÉ. Contents EDITOR'S PREFATORY NOTE AUTHOR'S PREFACE TABLE OF CONTENTS CHAPTER I THE EVOLUTION OF PHYSICS Revolutionary change in modern Physics only apparent: evolution not revolution the rule in Physical Theory-- Revival of metaphysical speculation and influence of Descartes: all phenomena reduced to matter and movement-- Modern physicists challenge this: physical, unlike mechanical, phenomena seldom reversible--Two schools, one considering experimental laws imperative, the other merely studying relations of magnitudes: both teach something of truth--Third or eclectic school-- Is mechanics a branch of electrical science? CHAPTER II MEASUREMENTS § 1. Metrology: Lord Kelvin's view of its necessity-- Its definition § 2. The Measure of Length: Necessity for unit-- Absolute length--History of Standard--Description of Standard Metre--Unit of wave-lengths preferable--The International Metre § 3. The Measure of Mass: Distinction between mass and weight--Objections to legal kilogramme and its precision--Possible improvement § 4. The Measure of Time: Unit of time the second--Alternative units proposed--Improvements in chronometry and invar § 5. The Measure of Temperature: Fundamental and derived units--Ordinary unit of temperature purely arbitrary--Absolute unit mass of H at pressure of 1 m. of Hg at 0° C.--Divergence of thermometric and thermodynamic scales--Helium thermometer for low, thermo-electric couple for high, temperatures--Lummer and Pringsheim's improvements in thermometry.