464 STANDARDISATION AND CALIBRATION OF APPARATUS. 



It is best to weigh the series 0-5, 0-2, 0-1, 0-1', 0-05, 0-02, 0-01, and 

 0-Or against the 1, 1', and I" weights, and take the mean of the three 

 values so obtained. 



When the weights have been standardised, a table should be drawn up 

 in the above fashion (Table CLVIIL). 



IT. Burettes. Carefully clean out the burette with hot chromic acid 

 mixture and rinse well with distilled water. Place it in a situation where 

 sudden changes of temperature can be avoided, and fill it above the zero 

 mark with distilled water ; note the temperature of this, which should be 

 as near as possible 60 F. (15-5 C.). 



Weigh an empty weighing bottle provided with a stopper ; cut two 

 parallel slits about 2 inches long and three-eighths of an inch apart in a 

 card (a visiting card answers admirably), and bend this so that the burette 

 passes through the slits, the narrow strip being in front ; adjust this so 

 that the upper edge of the narrow strip is coincident with the graduation 

 next below the zero mark. Now carefully run out the water so that the 

 lower edge of the meniscus coincides with the zero mark, cork up the burette 

 and leave it for a few minutes ; after making sure that no alteration in 

 level has occurred, adjust the card to the graduation next below the 5 c.c. 

 mark, and run out slowly 5 c.c. into the weighing bottle. Weigh this and 

 subtract the weight of the empty bottle ; the difference will give the weight 

 of water occupying the volume between and 5. 



After making sure that the level has not changed, adjust the card to the 

 graduation next below the 10 c.c. mark and run out a further 5 c.c. into 

 the weighing bottle ; weigh again, and subtract the weight of the empty 

 weighing bottle ; the difference will give the weight of the water occupying 

 the volume between and 10. 



Repeat this process till the lowest mark on the burette is reached. 



The calibration of the burette should be repeated two or three times and 

 the mean values tabulated. 



With a finely-divided rule measure the lengths of the divisions to 5, 

 to 10, etc. ; multiply each of these lengths by the total weight of water 

 and divide by the total length, to obtain figures commensurate with the 

 weights of water. 



Now plot out on squared paper two curves, one taking the scale readings 

 as ordinates, and differences between scale readings and weights of water 

 as abscissae ; the other taking scale readings as ordinates, and differences 

 between scale readings and lengths of scale corrected as described above as 

 abscissae. 



If both curves are nearly straight, it shows that the burette is made 

 from a tube of uniform bore, and is correctly divided ; if the two curves 

 have a marked curvature, but coincide in form, it shows that the burette 

 is made from a tube of uniform bore, but incorrectly divided ; if the two 

 curves do not coincide it shows that the tube is not uniform in bore. 



Now, obtain the value of the weights of water contained in each 5 c.c., 

 to 5, 5 to "10, etc., by subtracting the weight contained in to 5 from 

 that contained in to 10, etc., and the value of the lengths in a similar 

 manner ; divide one value by the other and plot out the values so obtained 

 on squared paper, taking the mean scale readings (i.e., for the volume 

 to 5 take 2-5) as ordinates, and the values obtained by the division at 

 abscissae. This will give the curve of irregularity of bore ; if at any part 

 of the curve it is noticed that the irregularity is very gross, the volume 

 of each 1 c.c. should be obtained by weighing the water ; if the curve is 

 appreciably regular, it is evident that the errors of the burette must be 

 due to incorrect division, and very careful measurements of the lengths of 

 divisions intermediate between each 5 c.c. mark should be made ; and if any 

 very grave faults are found, the burette should be especially calibrated at 

 that point. It is better, however, not to use a burette of this description. 



