r point that needs good heed taking to, namely, in using
different coal-tar colours to produce some mixed effect, or give some
special shade, the colours to be so mixed must possess compatibility
under like circumstances. For example, if you want a violet of a very
blue shade, and you take Methyl Violet and dissolve it in water and then
add Aniline Blue also in solution, you find that precipitation of the
colour takes place in flocks. A colouring matter which requires, as some
do, to be applied in an acid bath, ought not to be applied
simultaneously with one that dyes best in a neutral bath. Numerous
descriptions of methods of using coal-tar dyestuffs in hat-dyeing are
available in different volumes of the _Journal of the Society of
Chemical Industry_, and also tables for the detection of such dyestuffs
on the fibre.

Now I will mention a process for dyeing felt a deep dead black with a
coal-tar black dye which alone would not give a deep pure black, but one
with a bluish-purple shade. To neutralise this purple effect, a small
quantity of a yellow dyestuff and a trifle of indigotin are added. A
deep black is thus produced, faster to light than logwood black it is
stated, and one that goes on the fibre with the greatest ease. But I
have referred to the use of small quantities of differently coloured
dyes for the purpose of neutralising or destroying certain shades in the
predominating colour. Now I am conscious that this matter is one that is
wrapped in complete mystery, and far from the true ken of many of our
dyers; but the rational treatment of such questions possesses such vast
advantages, and pre-supposes a certain knowledge of the theory of
colour, of application and advantage so equally important, that I am
persuaded I should not close this course wisely without saying a few
words on that subject, namely, the optical properties of colours.

Colour is merely an impression produced upon the retina, and therefore
on the brain, by various surfaces or media when light falls upon them or
passes through them. Remove the light, and colour ceases to exist. The
colour of a substance does not depend so much on the chemical character
of that substance, but rather and more directly upon the physical
condition of the surface or medium upon which the light falls or through
which it passes. I can illustrate this easily. For example, there is a
bright-red paint known as Crooke's heat-indicating paint. If a piece of
iron coated wi