224 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
ple must be collected in such a manner as to avoid the 
contamination of one sample by another. The most sat- 
isfactory plan is to have a sufficient number of sterile 
pipettes or aluminum tubes of suitable size and to use 
a separate pipette or tube for each sample. (See method 
described on page 261.) The extension of the custom 
of delivering milk in bottles has greatly simplified the 
collection of samples for laboratory examination, espe- 
cially for the bacteriological and ferment tests. 
An instrument known as a “milk thief” is frequently 
used to collect samples of milk from vessels with straight 
sides, like shipping cans. It consists of a metal tube 
about 23 inches long with a handle at the upper end. 
This tube is inserted slowly into the can of milk in a 
perpendicular position until the lower end of the tube 
rests on the bottom of the can. The interior of the tube, 
of course, fills with milk. The opening at the top is 
closed tightly with the thumb, and the tube is then lifted 
out of the can, carrying with it the column of milk in 
the interior. The milk thus withdrawn may be emptied 
into a vessel by removing the thumb from the top of the 
tube. By this means, a column of milk extending from 
the bottom to the top of the fluid is obtained, which in- 
cludes all of the different strata existing at various levels 
in the can and which may therefore be regarded as a 
representative sample. Another type of tube sampler 
has a cap at the lower end with three openings, which 
are closed when the cap comes in contact with the bottom 
of the can. As the tube is pushed down into the can, 
the milk enters its interior through these openings and 
is held there when the cap is closed. The tube is then 
withdrawn and the milk emptied into a vessel. 
Preserving Samples—When the samples are to be 
