ldehyde_ (Hehner).

To 10 c.c. milk in a test tube add 5 c.c. concentrated _commercial_
sulphuric acid slowly, so that the two fluids do not mix. Hold the tube
vertically and agitate very gently. _Violet zone_ at the junction of the
two liquids = formaldehyde.

4. _Hydrogen Peroxide._

To 10 c.c. milk (diluted with equal quantities of water) in a test tube
add 0.4 c.c. of a 4 per cent. alcoholic solution of benzidine and 0.2
c.c. acetic acid. _Blue coloration_ of the mixture = hydrogen peroxide.

5. _Salicylic Acid._

Precipitate the caseinogen by the addition of acetic acid and filter. To
the filtrate add a few drops of 1 per cent. aqueous solution of ferric
chloride. _Purple coloration_ = salicylic acid.

6. _Sodium Carbonate or Bicarbonate._

To 10 c.c. of the milk in a test tube add 10 c.c. of alcohol and 0.3
c.c. of a 1 per cent. alcoholic solution of rosolic acid. _Brownish_
color = pure milk; _rose_ color = preserved milk.

[Illustration: FIG. 211.--Milk-collecting bottle and dipper in case.]

Quantitative.--

_Collection of Sample._--

The apparatus used for the collection of a retail mixed milk sample
consists of a cylindrical copper case, 16 cm. high and 9 cm. in
diameter, provided with a "pull-off" lid, containing a milk dipper, also
made of copper; and inside this, again, a wide-mouthed, stoppered glass
bottle of about 250 c.c. capacity (about 14 cm. high by 7 cm. diameter),
having a tablet for notes, sand-blasted on the side. The copper cylinder
and its contents, secured from shaking by packing with cotton-wool, are
sterilised in the hot-air oven (Fig. 26).

When collecting a sample,

1. Remove the cap from the cylinder.

2. Draw out the cotton-wool.

3. Lift out the bottle and dipper together.

4. Receive the milk in the sterile dipper, and pour it directly into the
sterile bottle.

5. Enter the particulars necessary for the identification of the
specimen, on the tablet, with a lead pencil, or pen and ink.

6. Pack the apparatus in the ice-box for transmission to the laboratory
in precisely the same manner as an ordinary water sample.

"Whole" milk may with advantage be collected in the sterile bottle
directly since the mouth is sufficiently wide for the milker to direct
the stream of milk into it.

~Condensed milk~ must be diluted with sterile distilled water in
accordance with the directions printed upon the label, then treated as
ordinary milk.

_Apparatus Required_:

C