With regard to the source of these coal-tar colours,
the word "coal-tar," I was going to say, speaks volumes, for the
destructive and dry distillation of coal in gas retorts at the highest
temperatures to yield illuminating gas, also yields us tar. But, coal
distilled at lower temperatures, as well as shale, as in Scotland, will
yield tar, but tar of another kind, from which colour-generating
substances cannot be obtained practically, but instead, paraffin oil and
paraffin wax for making candles, etc. Coal-tar contains a very large
number of different substances, but only a few of them can be extracted
profitably for colour-making. All the useful sources of colours and dyes
from coal-tar are simply compounds of carbon and hydrogen--hydrocarbons,
as they are called, with the exception of one, namely, phenol, or
carbolic acid. I am not speaking here of those coal-tar constituents
useful for making dyes, but of those actually extracted from coal-tar
for that purpose, _i.e._ extracted to profit. For example, aniline is
contained in coal-tar, but if we depended on the aniline contained ready
made in coal-tar for our aniline dyes, the prices of these dyes would
place them beyond our reach, would place them amongst diamonds and
precious stones in rarity and cost, so difficult is it to extract the
small quantity of aniline from coal-tar.
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