Quantitative chemical analysis : adapted for use in the laboratories of colleges and schools / by Frank Clowes and J. Bernard Coleman.

  • Frank Clowes
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Quantitative chemical analysis : adapted for use in the laboratories of colleges and schools / by Frank Clowes and J. Bernard Coleman. Source: Wellcome Collection.

    38/648
    13. Absolute Weight.—Since the volume of the body weighed usually differs from tliat of the weights which counterpoise it, different volumes of air are displaced by the substance and by the weights. Hence a body which has been counterpoised by weights in air would not usually remain in equilibrium with the same weights when the balance is in vacuo. The difference betM'een the result obtained by weighing a body in air and that obtained by weighing it in vacuo is usually so slight, that it may be neglected in ordinary analytical o])erations. The absolute weight of the substance in vacuo can be indirectly obtained, if necessary, in the following manner :—The weight of the volume of air displaced by the weights must be subtracted from the weight of the volume of air displaced by the substance, and the number thus obtained must be added to or subtracted from the weight on the scale-pan, according as the difference is positive or negative. Directions for Weighing. 14. The Following Rules slioiild be attended to during the operation of weighing :— 1. After the hahance has been levelled (7), ascertain that the scale-pans and the floor of the balance-case are perfectly clean. If necessary, carefully cleanse these surfaces with a large camel's-hair brush, which should be kept in the balance- case for the purpose. 2. Then gently release the pans and the beam (7), and if the beam is not caused to oscillate by its release, start its motion by a fanning movement of the hand over one of the pans. The pointer must oscillate through equal spaces on each side of the zero of the scale (7). This preliminary adjustment is unnecessary if the weight of the substance is obtained by difference or by substitution. Thus, if it is required to weigh out accurately about one gram of a powdered solid, the solid is put into a weighing bottle, or into some other convenient vessel, and the bottle and the substance are accurately weighed together. About a gram of the powder is then shaken out, and the vessel is again weighed. The difference between the first and