those shown in
Figs. 42 and 43, these tubes exhibit homogeneous curdling
of the milk without any subsequent digestion or
peptonisation of the coagulum. This is indicative of the
purity of the culture.]

The Society "Le Ferment" of Paris, which has been authorised by
Professor Metchnikoff to prepare and supply to the public his sour milk
culture, provides an apparatus for the treatment of the milk. It is
shown in Fig. 45, which consists of a double box having the intervening
space packed with a non-conducting material. It is provided with a
tight lid. Inside, there is accommodation for two milk vessels, each
with a capacity of about two thirds of a pint. The most difficult thing
in the souring of milk is to maintain the temperature as nearly at 100 deg.
F. as possible while the culture is in action. This result is attained
by filling the vessel in the middle with boiling water. The insulated
walls hinder the escape of heat, and the quantity of boiling water used
is calculated to maintain the temperature steady for the twelve hours of
cultivation; but in cold, frosty weather it is necessary to refill the
central vessel with boiling water in the middle of the period.

[Illustration: FIG. 45]

The milk jars are washed with hot boiled water and turned upside down to
dry. They should not be wiped with a cloth. Boil the milk to be treated
for ten minutes, stirring it to promote evaporation, as it is
advantageous to have it in concentrated form. Cool rapidly to 100 deg. F.
by placing the boiling vessel in cold water, add to each bowl one third of
a tube of the culture in powder form, fill up with the boiled and cooled
milk, stir well and cover. Place the jars in the box and fill the
central vessel with boiling water, shut the lid tight, and do not open
it (unless a fresh charge of boiling water is needed) for ten or twelve
hours, when it will be ready for use. If the liquid culture is used (one
small phial for each bowl) the milk should be cooled to 86 deg. F. instead
of 100 deg. F., as with the powder. The culture is also supplied in tabloid
form. The powder and tabloids keep well, but the liquid can only be
relied on for about two months. The milk prepared as above should be
stored in a cool place, the lids being kept on the bowls. It is good for
about two days, after which it becomes too sour. It can be eaten with
sugar, which not only sweetens it but is beneficial in affording
additional suitable food for the a