Ql'AXTITATirE AGRICULTURAL ANALYSIS
Standard Babcock Milk Pipette.—This pipette is graduated
to deliver 17.6 cc of water at 20° in 5 to 8 seconds.
CaHbration.—The official method for calibrating Babcock
test bottles is to fill the dry bottle to the zero mark with pure
mercury at 20°, weigh, fill to the highest mark and reweigh, cal-
culating the bulb and stem capacities on the basis of 13.5471
gin of dry mercury for eaAh cubic centimeter at 20°.
It is difficult to see what advantage this possesses over the
method of calibrating by weighing water at 20° especially since
the Babcock bottle filled with mercury must weigh more than
600 gin. Accurate weighing of such a quantity would require a
special balance, as sensitive as the analytical balance and having
large capacity.
Milk pipettes and graduates are calibrated according to the
official method by measuring in a burette the quantity of water
delivered by the instrument at 20°. Unless care has been exer-
cised in wetting the inner surface of the burette, using the
standard method by which the burette was calibrated, this
method will be subject to considerable error since all burettes
are graduated for delivery and not for capacity. A better
method for calibrating pipettes is described on page 46.
Determination of Fat: Babcock Method.—Fill a 17.6-cc pipette to the
mark with mixed milk sample and deliver to the graduated test bottle.
Add to this 17.5 cc of sulphuric acid (specific gravity 1.82 to 1.83), pouring
it in slowly so as to form a layer beneath the milk. Prepare an even number
of bottles, up to the capacity of the centrifuge. After the acid has been added
to all the bottles, mix the acid and milk by giving it a gentle rotary motion,
being careful to keep the liquid from collecting on the neck of the bottle.
Place the bottles in the centrifuge in such a way that they will be counter-
balanced and rotate for 4 minutes at the required speed for the machine
used. This is about 1,000 revolutions per minute for a wheel 10 inches in
diameter or 700 for a 24-inch wheel. Add hot water to each bottle until it is
filled to the neck and whirl 1 minute longer. Again add enough boiling
water to bring the fat column into the graduated portion of the neck and
whirl for another minute. Place the bottles in a glass vessel which is filled
with water at a temperature of 57° to 60°. The water should surround the
neck of the bottle to a point above the fat layer. After 1 minute measure
the fat column from the top of the upper meniscus to the plane of separation
between fat and aqueous solution, using a pair of dividers if considered
desirable.