ometer and Test Tube]

The simplest method of testing the acidity is to procure a few little
books of blue and red litmus test papers, and these can be had from any
philosophical instrument maker or laboratory furnisher. The strips of
test paper are torn out and dipped in the milk. When the milk is quite
fresh it will, owing to its amphoteric condition, change the red litmus
paper slightly blue, and the blue litmus paper slightly red. Old milk
changes blue litmus paper to a bright red because of its decided
acidity.

The above tests do not indicate if the milk is poor or rich, but this
can be determined by the lactometer, an instrument for ascertaining in a
simple way the specific gravity. The lactometer is shown in Fig. 7.

It is graduated usually from 25 deg. to 36 deg., corresponding to specific
gravities 1.029 to 1.038. It is graduated to degrees and half degrees.
Sometimes a thermometer is combined with the instrument. The specific
gravity rises as the temperature is lowered and decreases with increase
of temperature, so that it is important to make the test at the figure
at which the lactometer was graduated, which is usually 60 deg. F. Failing
this, an allowance has to be made for higher or lower temperatures. The
milk to be tested is well mixed, and placed in a deep vessel, and the
lactometer placed in it, holding it at first at an angle. It stands
upright and remains deeper or higher according to the specific gravity.
The reading is taken on the stem at the level of the milk. As the latter
is drawn up a little round the stem, about a half degree should be added
on to get the true figure. Thus, if the apparent reading is 31, the true
reading may be taken as 31.5. This is the average figure for good milk,
corresponding to a specific gravity of 1.0315; anything above this is
all to the good. Lower readings mean inferior quality, the latter being
proportionate to the lowness of the readings. The tests are most
conveniently made in a glass cylinder (Fig. 7), which may be purchased
with the lactometer. As there are many inaccurate instruments in the
market, it is necessary to go to a reputable maker, because an
unreliable lactometer is worse than useless.

The following table gives, in a condensed form, the allowances to be
made when the temperature is above or below the standard (60 deg. F.):

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Temperature.| Reading of Lacto