grade soaps, where the
perfumes added are necessarily more delicate and costly, is to make the
addition of the perfume when the colour has been thoroughly mixed
throughout the mass. Another method is to mill once and transfer the
mass to a rotary mixing machine, fitted with internal blades, of a
peculiar form, which revolve in opposite directions one within the other
as the mixer is rotated. The perfume, colouring matter, etc., are added
and the mixer closed and set in motion, when, after a short time, the
soap is reduced to a fine granular condition, with the colour and
perfume evenly distributed throughout the whole. By the use of such
machines, the loss of perfume by evaporation, which during milling is
quite appreciable, is reduced to a minimum, and the delicacy of the
aroma is preserved unimpaired.

[Illustration: FIG. 22.--Milling machine.]

Prolonged milling, especially with a suitable soap base, tends to
produce a semi-transparent appearance, which is admired by some, but the
increased cost of production by the repeated milling is not accompanied
by any real improvement in the soap.

_Perfume._--The materials used in perfuming soap will be dealt with
fully in the next chapter. The quantity necessary to be added varies
considerably with the nature of the essential oils, and also the price
at which the soap is intended to be sold. In the cheaper grades of
milled soaps the quantity will range from 10-30 fluid ozs. per cwt., and
but rarely exceeds 18-20 ozs., whereas in more costly soaps as much as
40-50 fluid ozs. are sometimes added to the cwt.

_Colouring Matter._--During recent years an outcry has been made against
highly coloured soaps, and the highest class soaps have been
manufactured either colourless or at the most with only a very delicate
tint. It is obvious that a white soap guarantees the use of only the
highest grade oils and fats, and excludes the introduction of any rosin,
and, so far, the desire for a white soap is doubtless justified. Many
perfumes, however, tend to quickly discolour a soap, hence the advantage
of giving it a slight tint. For this purpose a vegetable colouring
matter is preferable, and chlorophyll is very suitable.

[Illustration: FIG. 23.--Compressor.]

A demand still exists for brightly coloured soaps, and this is usually
met by the use of coal-tar dyes. The quantity required is of course
extremely small, so that no harm or disagreeable result could possibly
arise from the