52 Testing Milk and Its Products. 



ing of mercury will thus suffice for a number of calibrations. 

 In transferring the mercury, care must be taken that none of it 

 is lost, and that small drops of mercury are not left sticking to 

 the walls of the bottle emptied. A sharp tap on the bottle 

 with a lead pencil will help to remove minute drops of mercury 

 from the inside. Unless the bottles to be calibrated are per- 

 fectly clean and dry, it is impossible to transfer all the mer- 

 cury from one bottle to another. 



After several calibrations have been made, the mercury should 

 be weighed again in order to make certain that none has been 

 lost by the various manipulations. Scales similar to those shown 

 under (91) are sufficiently delicate for making these weighings. 



58. Test bottles may also be calibrated with mercury by weigh- 

 ing the bottles filled with mercury to the zero mark, and again 

 when filled to the 10 mark. This is the official method for test- 

 ing bottles adopted by the Association of Official Agricultural 

 Chemists (see 306). 



59. Cleaning mercury. Even with the best of care, mercury 

 used for calibration of glassware will gradually become dirty, 

 so that it will not flow freely over a clean surface of glass. It 

 may be cleaned from mechanical impurities, dust, grease, water, 

 etc., by filtration through heavy filter paper. This is folded in 

 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 

 graniteware dish and the nitric acid poured over it, the dish 

 being covered to keep out dust. The acid solution is then care- 

 fully poured off and the mercury washed with water; the latter 

 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 mer- 

 cury are to be made. 



Mercury forms the most satisfactory and accurate material 

 for calibration of test bottles, on account of its heavy weight 



