into
consideration in regulating the temperature and flow of air through the
drying chamber, and for this purpose the use of a hygrometer is
advantageous.

It is very important that the correct amount of moisture should be left
in the soap, not too much, nor too little; the exact point can only be
determined by judgment and experience, and depends to a considerable
extent upon the nature of the soap, and also on the amount of perfume or
medicament to be added, but speaking generally, a range of 11 to 14 per
cent. gives good results. If the soap contains less than this amount it
is liable to crumble during the milling, will not compress
satisfactorily, and the finished tablet may have a tendency to crack and
contain gritty particles so objectionable in use. If, on the other hand,
the soap is left too moist, it is apt to stick to the rollers and mill
with difficulty, and during compression the surface assumes a blistered
and sticky appearance.

(ii.) _Milling and Incorporation of Colour, Perfume or Medicament._--The
object of milling is to render the soap perfectly homogeneous, and to
reduce it to a state in which colour, perfume, or any necessary
neutralising material or other substance may be thoroughly incorporated.
The milling machine consists of smooth granite rollers, fitted with
suitable gearing and working in an iron framework (Fig. 22). The rollers
are connected in such a manner that they rotate at different speeds, and
this increases the efficiency of the milling, and ensures that the
action of the rollers is one of rubbing rather than crushing.

By means of suitably arranged screws the pressure of the rollers on one
another can be adjusted to give the issuing soap any desired thickness;
care should be taken that the sheets of soap are not unnecessarily thick
or the colour and odour will not be uniform.

The soap, in the form of chips, is introduced on to the rollers through
a hopper, and after one passage through the mill, from bottom to top,
one of the serrated knife edges is applied and the ribbons of the soap
are delivered into the top of the hopper where the colour, perfume, and
any other desired admixture is added, and the milling operation repeated
three or four times. When the incorporation is complete the other
scraper is fixed against the top roller and the soap ribbon passed into
the receptacle from which it is conveyed to the compressor. A better
plan, however, especially in the case of the best