cidify with acetic
acid and add lead acetate, a yellow precipitate indicates the presence
of chrome.

A book on qualitative chemical analysis should be referred to for
further details and tests for metallic mordants.

The fastness of colours to light, air, rubbing, washing, soaping,
acids and alkalies is a feature of some considerable importance, there
are indeed few colours that will resist all these influences, and such
are fully entitled to be called fast. The degree of fastness varies
very considerably, some colours will resist acids and alkalies well,
but are not fast to light and air; some will resist washing and
soaping, but are not fast to acids; some may be fast to light, but are
not so to washing. The following notes will show how to test these
features.

#Fastness to Light and Air.#--This is simply tested by hanging a piece
of the dyed cloth in the air, keeping a piece in a drawer to refer to,
so that the influence on the original colour can be noted from time to
time. If the piece is left out in the open one gets not only the
effect of light but also that of climate on the colour, and there (p. 222)
is no doubt rain, hail and snow have some influence on the fading of
the colour. If the piece is exposed under glass the climatic
influences do not come into play, and one gets the effect of light
alone.

In making tests of fastness the dyer will and does pay due regard to
the character of the influences that the material will be subjected to
in actual use, and these vary very considerably; thus the colour of
underclothing need not be fast to light, for it is rarely subjected to
that agent of destruction; on the other hand, it must be fast to
washing, for that is an operation to which underclothing is subjected
week by week.

Window curtains are much exposed to light and air, and, therefore, the
colours in which they are dyed should be fast to light and air. On the
other hand, these curtains are rarely washed, and so the colour need
not be quite fast to washing. And so with other kinds of fabrics;
there are scarcely two kinds which are subjected to the same
influences and require the colours to have the same degree of
fastness.

The fastness to rubbing is generally tested by rubbing the dyed cloth
with a piece of white paper.

#Fastness to Washing.#--This is generally tested by boiling a swatch of
the cloth in a solution of soap containing 4 grammes of a good neutral
curd soap per litre for ten minute