More than six years ago, whilst collecting together the results of the most trustworthy measurements of the various physical constants, with a view of establishing certain relationships which I conceived to exist between them, I was much struck with the discrepancies which exist, not only between the observations of different experimenters, but also frequently between those of the same individual. Many of these discrepancies, no doubt, arise from differences in the purity of the substances employed; but, when all due allowance has been made for such a cause, there still exists a large margin, which can only be accounted for by assuming that mere alteration of molecular aggregation must modify always, and in many cases considerably, the action of physical forces. This assumption has been already abundantly justified by the experimental researches of several eminent philosophers in every part of the domain of physical science; but, though much has been done, very much more remains to be done, and a wide field lies open before us which cannot fail to richly reward the patient explorer. I write patient explorer advisedly, because, in many cases, any difference of molecular disposition which we can effect with the means at our disposal, produces only a very minute alteration in that particular physical quality which we may be examining, and an alteration which we can only hope to measure accurately by the exercise of a large amount of perseverance and conscientious labour. But whether the effect to be observed be large or small, there is no doubt that further investigations of the kind here indicated must be made ere we can gain a true insight into the nature of the action of physical forces or into any relationships which may exist between them. I proposed to myself, therefore, to examine as far as possible each of the various physical properties of one and the same specimen of different kinds of matter; and, further, to investigate the alterations which can be produced in these properties by stress and strain; being convinced that, by so doing, much light would be thrown on such subjects as electrical conduction, magnetic induction, thermal conduction and expansion, thermo-electricity, specific heat and elasticity. The words “stress” and “strain” are here used in their widest sense as intending to denote respectively the equilibrating application of any physical force, and the definite alteration of form or dimensions experienced thereby by the matter acted on by the force.