MILK AND POTATOES 75 



solution is obtained. Filter, and preserve in a flask 

 stoppered with cotton-wool, after sterilising by boiling 

 for half an hour on two successive days. For some 

 purposes a special solution of litmus, the Kubel-Tiemann 

 solution, which can be procured ready for use, is employed. 

 It must not have any antiseptic added to it (as is some- 

 times done to preserve it for use in the chemical 

 laboratory). 



Sufficient of this litmus infusion is added to the nutrient 

 media, after filtration, to tinge them a distinct purplish 

 colour. After steaming the colour has usually dis- 

 appeared, but returns as the tubes cool. 



Milk. Use separated milk, but failing this, centri- 

 fugalise ordinary new milk, or place it in a tall cylinder 

 and allow it to stand overnight in a cool place, preferably 

 in an ice safe. Then pipette off the milk from the bottom, 

 rejecting the cream. Introduce the separated milk into 

 test-tubes to the depth of about an inch to an inch and 

 a half and steam for an hour on two successive days. The 

 milk is usually tinged with litmus before tubing, forming 

 litmus milk. 



Potatoes. Choose sound potatoes, and scrub them well 

 with water to remove dirt. Cut off the ends, and with a 

 cork-borer, slightly smaller than the test-tubes which are 

 used, bore through the potato so that a cylindrical piece 

 is removed. Push this out of the borer, and divide it 

 into two portions by a very oblique transverse cut, so 

 that two wedge-shaped pieces are obtained, and in this 

 manner prepare as many pieces as there are tubes to be 

 filled. Place them in a basin under the tap, and allow 

 the water to flow over them for about two hours. This 

 prevents the darkening of the potato in the subsequent 

 steaming, as does also the use of a silver borer. The 

 test-tubes for the potato-wedges are prepared as follows : 

 After plugging and sterilising in the ordinary way, intro- 



