THE ANALYSIS OF MILK 21 



Hold the bottle by the stem, and by the cork, keeping 

 a slight pressure on the cork, and mix the contents by 

 shaking ; when all the white particles of curd have dis- 

 appeared, but not before, invert the bottle to allow the 

 acid to run out of the neck, and then turn it upright ; 

 repeat this several times till the acid that was in the 

 neck has completely mixed. The bottle is now almost 

 too hot to hold, from the heat developed by the action 

 of the acid, and its colour is brown ; place it in the 

 centrifuge. Treat the other bottles in the same way, 

 and arrange them in order in the centrifuge. It is well 

 to mark each cup with a number to avoid errors. 



The shaking of all the bottles may be done at once 

 by means of the author's stand (Fig. 16), in which the 

 bottles are held by being pushed into slits in the india- 

 rubber plate ; the hand should be placed over all the 

 corks to prevent them from coming out, and the con- 

 tents of the bottles from being spilt. Other convenient 

 stands are also made. 



If a number of samples insufficient to fill the disc 

 is being tested, care should be taken to place them 

 symmetrically so as to preserve the balance of the 

 machine ; a bottle filled with a mixture of equal parts of 

 acid and water may be kept to make up an even number 

 of bottles should an odd number of samples be tested. 



Screw on the cover, and rotate the machine at about 

 1000 revolutions per minute for three or four minutes ; 

 modern centrifuges are usually turned with a handle ; 

 if there is a plain spindle, with a projecting piece, 

 place the eye of the cord on the projection and turn 

 the machine round counter-clock-wise till the cord is 

 wound round the spindle ; take the handle in the 

 hand, pull hard, and the disc will spin ; usually this 

 must be done twice. If there is a strap, place this half 

 round the pulley, and pull with the right hand in a 

 somewhat downward direction, keeping the strap taut 

 with the left hand, and continue giving sharp pulls till the 

 disc spins rapidly ; the pull must be downward as well 

 as forward, or the pulley will not engage the spindle ; 



