6 BULLETIN" 240, U. S. DEPARTMENT OF AGRICULTURE. 
cool for a few minutes in air until the cap becomes concave, as this 
is said to hold the cap on tight and helps to make it water-tight. 
Obviously, it is of utmost importance that the caps be water-tight, 
since they are submerged in water during cooling, and if not tight 
the milk may become infected by polluted cooling water. 
When bottles are submerged the ordinary cardboard cap is of no 
value for pasteurization in the bottle, since water will easily pene- 
trate during cooling. This makes it necessary to use some form of 
patented cap, of which both specially treated cardboard and metal 
caps are on the market. It is almost needless to state that if the edpre 
of the bottle is chipped or otherwise imperfect almost any seal cap 
will not be water-tight during the cooling. Imperfect bottles must 
not be used. It is claimed by the manufacturers of patented seal 
150° F. 
140 F. 
130° F. 
1 20° F. 
no°F. 
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90 F. 
8o°F, 
70°F. 
60° F. 
50°F. 
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Heating period in minutes. 
-Variations in temperature in different parts of quart bottles of water during the 
process of pasteurization in the bottle. 
caps that they are tight on perfect bottles. It would be advisable, 
however, for the dairyman to test the tightness of his caps by the 
following method : Fill the milk bottle with a 0.05 per cent solution 
of barium chlorid (BaCL). The barium-chlorid solution should be 
made up with distilled water, since the sulfates present in ordinary 
water will cloud the solution. Cap the bottles in the usual way 
with a seal cap and heat to 145° F., submerge, and cool in a 10 per 
cent solution of magnesium sulfate (MgSo 4 ). 
If any of the magnesium sulfate leaks into the bottle during cool- 
ing the barium-chlorid solution will become cloudy, owing to the 
formation of barium sulfate, which is insoluble. This test is very 
delicate and will show even a slight leak. Both these chemicals may 
be obtained at any drug store. Since barium chlorid is poisonous, 
after testing bottles in which it has been used care must be taken to 
