§ 41. MEASURING AND DIVIDING SOLUTIONS. 41 



The analyst should test tlie correctness of the gradua- 

 tion of his instruments before using them, by comparing 

 them with each other; the '|^ litre flask should require 

 just as much water to fill it twice up to the mark on the 

 neck as is required to fill the ' \^ litre flask once up to the 

 mark on its neck. In the same way the ' 1^ litre flask 

 should be compared with the 1 litre flask, and these with 

 the graduated cylinders, and the pipettes with each other 

 and the graduated cylinders. 



When a certain quantity of any standard or titrated 

 solution is to be measured out with a pipette or flask, the 

 instrument should either be dry on tlie inside, or it should 

 be rinsed out with a little of the solution to be measured, 

 and the last drop of the solution that remains in the point 

 of the pipette should either always be allowed to remain 

 there, or it should always be blown out into the vessel 

 containing the measured solution ; the same course should 

 be followed in testing the graduation of the pipettes. 



To read ofl" the lieight of a solution in a burette or 

 other graduated instrument, be sure, first, that it is in a 

 vertical position, so that the surface of the liquid in it 

 will be horizontal ; then place the cylinder between the 

 eye and a brightly illuminated white wall, and read the 

 height of the lower surface of the dark zone that is read- 

 ily seen under these circumstances just beneath the sur- 

 face, while the eye is in the same horizontal plane. 



In filling a Mohr's burette, fill up to above the zero 

 mark with the solution, and quickly open wide the clamp 

 for a moment so that the rubber tube and the glass tube 

 below the clamp will be completely filled ; then open the 

 clamp a little and allow the liquid to flow out, drop by 

 drop, until the dark zone, mentioned above, reaches the 

 zero mark. 



The temperature of all measured liquids should be as 

 nearly 15° C. as possible. 



When the quantity of a solution to be divided is not 



