Cohesion

Abstract

WHEN one turns from an account of the discovery of a “dark star” by celestial dynamics to an investigation of the properties of the excessively minute whirling electrons in an atom, the impression is gained that within these limits at least there is but little more than relatively unimportant detail to learn. Such a notion is quite erroneous. More is known of the mechanism of plants on one hand and of electrons on the other than of the most ordinary and apparently simple mechanical phenomena. The most expert physicist can make only a near guess as to the motion of a billiard ball under given conditions as to stroke, weight, etc., since there is an imperfectly known factor, friction, in the problem. Similarly, although he can calculate with great precision the force with which one piece of iron attracts another when they are a foot apart, he cannot say with any accuracy from first principles what is the tensile strength in each piece of iron. Engineers similarly have made countless experiments and have also obtained very many data from constructional experience which give average values from which, by allowing a liberal margin for uncertainty, structures can be safely designed; but that is all.