PREPARATION OF SMEARS o97 
which measures 1/100 c.c. The most convenient form of pipette is the 
straight capillary pipette, calibrated to deliver 1/100 
c.c., the graduation mark being 13 to 24 ins. from 
the tip. Such pipettes are now for sale by manu- 
facturers, and can be easily obtained. The cali- 
bration should be tested by weighing with chemical 
balances the amount of milk discharged from the 
tube. Only a single pipette is needed in making a 
series of tests, provided this is kept clean while in 
use. Inthis kind of work cleanliness rather than 
sterilization is required. Clean towels may be used 
for wiping the exterior of these pipettes, while their 
bores may be kept clean by rinsing them in clean 
water between each sample. The small amount of 
water left in the tube may be rinsed out into the 
milk sample under examination. This method of 
procedure, while adding a small number of bacteria 
to each sample, introduces only a theoretical error, 
tests showing that such bacteria cannot subsequently 
be detected, and make no difference in the final Pp 
result. 
Preparation of Smears. One one-hundredth c.c. 
of milk or cream is deposited upon a clean glass slide 
by means of a pipette described above. By the use 
of a clean, stiff needle this drop of milk is spread over 
an area of 1 sq.cm. This may be most conveniently 
done by placing the slide upon any glass or paper 
ruled into areas 1 cm. square. These marks show- 
ing through the glass serve as guides. After uniform 
spreading the preparation is dried in a warm place 
upon a level surface. In order to prevent noticeable A B 
growth this drying must be accomplished within Pye. 67.— Breed’s 
five to ten minutes; but excessive heat must be Capillary Pipettes. 
avoided or the dry films may crack and peel from the ‘Several types of pi- 
pettes have been de- 
slides in later handling. vised. Type A gives 
After drying, the slides are to be dipped in 
xylol (gasoline may be used) for one minute, then 
drained and the slides dried. They are then 
immersed in 90 per cent grain or denatured 
alcohol, for one minute or more, anJ then trans- 
ferred to a fresh aqueous solution of methylene 
as much satisfaction 
asany Pruchaatthe 
Hilinois Agricultural 
Station uses ordinary 
capillary tubing and 
standardizes the 
amount of milk deliv- 
ered by weighing it on 
an analytical balance. 
blue. Old or 
