The Babcock Test. 49 



become dirty, so that it will not flow freely over a clean 

 surface of glass. It may be cleaned from mechanical 

 impurities, dust, films of grease, water, etc., by filtration 

 through heavy filter paper. This is folded the usual 

 way, placed in an ordinary glass funnel and its point 

 perforated with a couple of pin holes. The mercury will 

 pass through in fine streams, leaving the impurities on 

 the filter paper. Mercury may be freed from foreign 

 metals, zinc, lead, etc., sometimes noticed as a grayish, 

 thin film on its surface, by leaving it in contact with 

 common nitric acid for a number of hours; the mercury 

 is best placed in a shallow porcelain or granite ware dish 

 and the nitric acid poured over it, the dish being covered 

 to keep out dust. The acid solution is then carefully 

 poured off and the mercury washed with water; the lat- 

 ter is in turn poured off, and the last traces of water 

 absorbed by means of clean, heavy filter paper. 



The mercury to be used for calibration of glassware 

 should be kept in a strong bottle, closed by an ordinary 

 stopper. In handling mercury, care must be taken not 

 to spill any portion of it; finger-rings should be removed 

 when calibrations with mercury are to be made. 



Mercury forms the most satisfactory and accurate ma- 

 terial for calibration of test bottles, on account of its 

 heavy weight and the ease with which it may be manip- 

 ulated. Equally correct results may, however, with 

 proper care be obtained by using water for the calibration. 



(B.) Calibration with water. This may be done by 

 means of a delicate pipette or burette, or by weighing in 

 a somewhat similar manner, as explained in case of cali- 

 bration with mercury. 

 4 



