Graduation and Calibration of Apparatus 151 



straight-edge up to it, lift the needle, maintaining constant 

 inclination, and mark the burette. Repeat the whole process 

 for each mark transferred. 



In either case the tenths have now to be rilled in. This 

 is accomplished by dividing a number of cubic centimetre 

 divisions on the small detached piece as before, and securing 

 it successively in convenient positions on the main scale with 

 two needles placed at the end division but one of the small 

 scale, and pricking through, in each case, exactly to a whole- 

 number mark upon the main scale. By this means the division 

 of the whole scale is avoided, and sufficient accuracy obtained. 



The whole divisions having been marked large scratches 

 for the whole numbers, smaller ones for 0*5 c.c., and smallest 

 for the 0*1 c.c. marks proceed to number from the top down- 

 wards, making the numbers as small as possible, and the shapes 

 distinct. See Fig. 113. 



1234567890 



FIG. 113. 



Remove the burette, wax all portions not intended to be 

 etched, mark if desirable initials and date of graduation, 

 paint all marks with a brush dipped in hydrofluoric acid 

 solution, so as to remove air-bubbles, and fix filter paper strips 

 dipped in hydrofluoric acid solution upon the marked portion 

 of the burette as before, making quite sure no air-bubbles 

 exist underneath the filter paper. After ten minutes, wash, 

 heat, remove the wax, clean up with benzene, rub the burette 

 with a mixture of white lead, plaster of Paris, and boiled linseed 

 oil until the etched marks stand out plainly ; wipe with a dry 

 cloth, and proceed with the calibration. 



Calibration is performed by filling the burette up to zero 

 mark with distilled water, noting the temperature before and after 

 the experiment, determining the density of water at the mean 

 temperature, and weighing successive 5 c.c. in a dry beaker, 

 or weighing bottle. The 5 c.c. must in each case be drawn 

 off slowly, as the burette will be used in this way. 



The weight divided by the density of the water will give the 



