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.

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    14.] DIRECTIONS FOR WEIGHING. n second Aveigliings will give the weiglit of the powder shaken out. In cases similar to this it is not necessary to attend to the previous adjustment of the beam to exact equilibrium. 3. The substance should he invariably placed on the left- liand pan, and the weights are then afterwards placed by the forceps on the right-hand pan. 'Jliis insures the con- venience of putting on, changing, and removing the weights by the right hand, the beam being released from its support by the left hand. 4. The weights should be placed on the pan in systematic order. It is best to commence with a weight which is judged to be somewhat too heavy. If this is found to be the case, lower Aveights of the same denomination are tried in succession, until equihbrium results or the weight is somewhat too small in amount. If the weight is too small the weights of the next lower denomination should be added to it in the same systematic order. This method will be found much more expeditious than that of selecting the weights at random. The milligrams and fractions of a milligram are usually ascertained by the use of the rider. 5. The near approach to equipoise of the substance and the weights is judged by observing the sAvings of the pointer over the graduated scale. Consecutive swings to the right and to the left must finally be equal in extent. • 6. The balance-case should be closed while the rider is being used; and the final observation of equihbrium must be always conducted with the case closed. 7. The weights must never be handled with the fingers, but should always be lifted with the forceps. 8. During the transfer of the weights, or the addition or removal of the body weighed, the motion of the beam and of the pans should invariably be arrested. This prevents the unnecessary wearing of the knife-edges by violent oscillation.