[15] From Proceedings of Royal Society, vol. XXV. p. 396.
[16] This may be done by clamping the tube which supplies mercury below C, exhausting A, and then opening the clamped tube and allowing the mercury to rise.
CHAPTER V.
GRADUATING AND CALIBRATING GLASS APPARATUS.
Although the subjects to which this concluding chapter is devoted do not, properly speaking, consist of operations in glass-blowing, they are so allied to the subject, and of such great importance, that I think a brief account of them may advantageously be included.
Graduating Tubes, etc.—It was formerly the custom to graduate the apparatus intended for use in quantitative work into parts of equal capacity; for example, into cubic centimetres and fractions of cubic centimetres. For the operations of volumetric analysis by liquids this is still done. But for most purposes it is better to employ a scale of equal divisions by length, usually of millimetres, and to determine the relative values of the divisions afterwards, as described under calibration. It rarely happens that the tube of which a burette or eudiometer is made has equal divisions of its length of exactly equal capacities throughout its entire length, and indeed, even for ordinary volumetric work, no burette should be employed before its accuracy has been verified. An excellent method for graduating glass tubes by hand[17] has been described in Watts’s Dictionary of Chemistry, and elsewhere. Another excellent plan, which I have permission to describe, has been employed by Professor W. Ramsay. It will be sufficient if I explain its application to the operation of graduating a tube or strip of glass in millimetre divisions.
The apparatus required consists of a standard metre measure,[18] divided into millimetres along each of its edges, with centimetre divisions between them, a ruler adapted to the standard metre, as subsequently explained, and a style with a fine point for marking waxed surfaces.