162 UNIVERSITY OF WISCONSIN STUDIES 
change in staining reaction between the leucocytes of raw 
and pasteurized milk takes place at the temperature of 63° C. 
When milk is subjected to this temperature for varying 
lengths of time it is found that the definite staining reaction 
takes place when the temperature is maintained for about 
twenty minutes. When the time of exposure is eighteen min- 
utes or less, some of the leucocytes do not take the stain in 
the characteristic way. So it may be stated that the character- 
istic staining reaction occurs when the milk is subjected to 
a temperature of 60° to 68° C. for upwards of twenty minutes. 
Effect of stain on bacteria.—The heat which fixes the 
nuclei of the leucocytes for staining curiously renders the 
bacteria less likely to take the stain. It is only in raw milks 
then that the bacteria stain well. In well pasteurized milks 
they stain poorly. Frequently the bacteria take the stain with 
a varying degree of intensity, but whether this is due to in- 
herent differences in the bacteria or to the effect of the heat 
on them is undetermined. The reaction of the bacteria to 
the stain has up to now shown illogical variations, but in 
general the following conclusions seem warranted. The 
bacteria in raw milk are always stained, usually well and 
sometimes very beautifully. In the sediment from properly 
pasteurized milk examined within a few hours after pasteuri- 
zation the bacteria are usually invisible. Old pasteurized 
milk may show the nuclei of the cells well stained and the 
bacteria also. In this case we assume that the bacteria which 
stain have been introduced or grown subsequent to the pro- 
cess of pasteurization. 
THe MicroscoricaL PICTURE OF A PROPERLY PASTEURIZED MILK 
When a milk has been stained by the Wet Process described 
in this paper and the sediment mounted without the addition 
of water is examined under the microscope, the following 
picture will be obtained if it has been pasteurized at a tem- 
perature of 63° C. (145° F.) or above, and held at that 
temperature for twenty minutes or more. The nuclei of prac- 
tically all of the leucocytes are in compact masses. They 
are well stained. The background is lighter than the nuclei 
