METHODS OF TESTING BABCOCK MILK BOTTLES. 133 



II. Gravimetric Method, using Distilled Water. 



The bottle is filled up to the zero mark with water, the 

 neck dried carefully as in Method I, and the bottle then weighed. 

 Next, water is added to exactly the 4 per cent, mark, and the 

 bottle and contents again weighed, this being repeated by pouring 

 water up to the top of the scale, care being taken in each case 

 to have the neck above the water level as dry as possible. From 

 the weight of water added each time, and the temperature, the 

 exact volume is found, and this is converted into percentages. 

 In this way, the error up to the 4 per cent, mark, and also for 

 the whole scale, is obtained. 



111. Gravimetric Method, using ^Mercury. 



In this method, mercury is run in from a separating funnel 

 up to approximately the zero mark, the exact reading being taken 

 by means of a table cathetometer, and the bottle weighed. 

 Mercury is next run in to approximately the middle graduation 

 (4 per cent.), and finally up to approximately the top graduation 

 (8 per cent.), the weight run in being ascertained in each case. 

 The error in the graduated scale is then obtained (as in Method 

 II) for approximately the bottom half and over approximately 

 the whole scale. 



The errors in the graduated scale at these two points, 

 obtained by each of the above methods for ten bottles, are given 

 in the following table. The error is taken to be a negative one 

 when the true reading is less than the apparent one on the scale. 



On comparing the results (see Table) obtained by the various 

 methods it will be noticed : — 



1. Duplicate results by the volumetric method compare fairly 

 closely as a rule, but those obtained by the two gravimetric 

 methods are rather erratic, especially some obtained by the thiin 

 method (using mercury). 



2. The results obtained by the second method are uniformly 

 lower than those obtained by the other two methods, thus causing, 

 in these particular bottles, the errors (being negative) to be 

 greater than those obtained by the other methods. This may 

 possibly be explained by the initial weighing in the second 

 method being generally too high, owing to the film of moisture 

 left on the neck of the bottle in spite of the precautions that 

 were taken to wipe off the superfluous moisture previously. In 

 most cases, the mean errors obtained by the first and by the 

 third methods compare fairly well. 



The lack of concordance in the results obtained in repeated 

 calibrations of the same bottle, especially by the two gravimetric 

 inethods, may be due to any of the following sources of error in 

 working : — 



1. The closeness of the graduation marks: The distance 

 between each line (equivalent to 0*1 per cent.) is less than 1 mm. 

 in length, so that it is veiy difficult to read to a close approxima- 

 tion of the second place of decimals, even by the use of a table 

 cathetometer. 



