THE BABCOCK TEST 47 



the tester shows the 5 and 10 per cent, marks to be 

 correct. 



In explanation of the use of this form of bottle-tes- 

 ter, it is to be remembered that the neck of the milk- 

 bottle is so graduated as to hold 2 cc. between the o 

 and 10 marks ; hence, the volume between the o and 

 5 marks should be i cc, and that between the 5 and 

 10 marks should be also i cc. The brass plunger is 

 so made that each section displaces, or forces up into 

 the neck, i cc. of liquid, the whole instrument displac- 

 ing 2 cc. This tester therefore gives two tests of the 

 scale, one at the 5 per cent, mark and the other at 

 the 10 mark. 



Some of these instruments are made to test the 4 

 and 8 per cent, points, so that with two testers, one 

 can, if desired, test the accuracy of the scale at the 

 4, 5, 8 and 10 points. There are also testers of the 

 same form made for cream-bottles. 



In using this bottle-tester, the following precautions 

 are to be observed : 



( 1 ) Have the upper surface of the liquid exactly 

 on a level with the zero mark in the neck of the test- 

 bottle before putting the tester in. 



(2) Clean the inside walls of the neck of the bot- 

 tle from adhering liquid before testing. 



(3) No air-bubbles should be allowed to adhere to 

 the tester when it is below the liquid. 



(4) The tester should be dry each time before using. 

 (2) Testing zvith mercury. — From an accurately 



graduated burette (Fig. 21), measure 2 cc. of clean 

 mercury into the bottle to be tested. Then push down 

 into the neck of the bottle as far as the top line of 



