8 



Press up or ease the stopper till the bottom of the column of fat 

 coincides with one of the longest marks. If now it is remem- 

 bered that, when 11 c.c. of milk have been used in the test, the 

 space between two long lines represents 1 per cent, of fat, and 

 that between two shorter lines 1 per cent., it will be found 

 extremely easy to read off the percentage of fat in any sample. 

 A careful reader should be able to read to 05 per cent. Head 

 from below upwards, and again from the top downward. A 

 very common source of error occurs at this juncture owing to 

 the column of fat moving its position during the time occupied 

 in taking the two readings. The top of the fat column is curved. 

 The bottom of the dark curve is the correct point to read when 

 whole milk is being tested. The fat should be free from any 

 appearance of bubbles at the bottom of the column. If bubbles 

 are present they indicate that the whirling was insufficient. 

 When a dirty layer or a layer of undissolved curd appears 

 between the fat and the acid solution it indicates that the acid 

 is too weak or that the bottle has not been sufficiently shaken 

 if black that the acid is too strong, that the milk has been 

 dropped into the acid instead of being poured down the side of 

 the bottle, that the bottle has been dirty, or that the stopper is 

 in bad condition. Bubbles on the surface of the fat may be got 

 rid of by giving the bottle a sharp tap on the table. If in 

 attempting to remove the bubbles, or if in moving the stopper 

 of the bottle while reading, any fat is spurted into the neck the 

 bottle must be whirled again. 



The marks on the test bottle are usually cut deeply into the 

 glass, and should be quite distinct. If they are not distinct, a 

 little black varnish should be rubbed over the neck, so as to make 

 the divisions stand out boldly. 



The india rubber stoppers should be reversible, and in order 

 that they may be securely held in position the neck of the test 

 bottle is usually moulded so that the inner surface resembles 

 the thread of a screw. 



For testing butter-milk or separated milk, a newer pattern 

 of bottle, called a "precision" bottle, has been invented. In 

 this form the upper portion of the stem is narrowed, and the 

 space between each division correspondingly lengthened so that 

 a small percentage of fat can easily be read. These bottles 

 should not, however, be used for testing whole milk. 



iii. BURETTES. 



Where a large number of samples have to be tested it will 

 be necessary to measure the sulphuric acid and the amyl alcohol 

 with burettes, and to discard the use of the pipettes supplied 

 for measuring these reagents. The burette for the sulphuric 

 acid should be replenished from a stock bottle placed on a stand 

 sufficiently low to prevent syphoning. A little practice will be 

 required to enable the operator to freely run oft 10 c.c. of acid 

 and 1 c.c. of amyl alcohol. 



