A course of instruction in quantitative chemical analysis for beginning students: With explanatory notes, questions and analytical problems

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921. Excerpt: ... I. WEIGHING1 Standards of Mass. The fundamental standard of mass adopted by the United States is a cylinder of platinum-iridium kept at the International Bureau of Weights and Measures near Paris. Two authentic copies of this standard, of the same form and composition, are kept in a vault at the National Bureau of Standards, and they are used only when needed to verify the secondary standards of the Bureau. Mass standards are usually called "weights," but it should be realized that, while the weight of a body depends upon its attraction by the earth, the mass of a body is a property inherent in the body itself. Since, however, the masses of bodies are proportional to their weights at the same locality, we may compare the masses by making a comparison of the respective forces of gravity on the bodies under consideration. The purpose of weighing, then, is to compare the quantity of matter in a specific object with the quantity of matter in a given standard--a gram or kilogram weight. The comparison is made on the balance, by means of the lever principle, by suspending the object at one end of a beam, and the weights at the other end of the beam, the beam being virtually a kind of lever. The Balance. The beam of the balance is supported on a central knife-edge, usually of agate, which rests upon a plane agate plate; and two pans for supporting the masses to be compared are vertically suspended from stirrups, each of which has an agate bearing which rests on a knife-edge fixed at one extremity of the beam. The arms of the balance are so graduated that a rider (of known weight) can be placed on the beam at any required distance from the central knife-edge. If the three knife-edges are allowed to press continually upon their agate bearings, they soon become bl...