rious components of the tissue for the different colouring-matters,
these components are capable of being differentiated and distinguished by
microscopical analysis. Furthermore, the almost invisible organisms which
in recent times have been shown to play such an important part in
diseases, have in many cases a special affinity for particular
colouring-matters, and their presence has been revealed by this means. The
micro-organism of tubercle, for example, was in this way found by Koch to
be readily stained by methylene blue, and its detection was thus rendered
possible with certainty. Many of the dyes referred to in the previous
pages have rendered service in a similar way. To the pure utilitarian such
an application of coal-tar products will no doubt compensate for any
defects which they may be supposed to possess from the aesthetic point of
view.[11]

From a small beginning there has thus developed in a period of
five-and-thirty years an enormous industry, the actual value of which at
the present time it is very difficult to estimate. We shall not be far out
if we put down the value of the coal-tar colouring-matters produced
annually in this country and on the Continent at L5,000,000 sterling. The
products which half a century or so ago were made in the laboratory with
great difficulty, and only in very small quantities, are now turned out by
the hundredweight and the ton.[12] To achieve these results the most
profound chemical knowledge has been combined with the highest
technological skill. The outcome has been to place at the service of man,
from the waste products of the gas-manufacturer, a series of
colouring-matters which can compete with the natural dyes, and which in
many cases have displaced the latter. From this source we have also been
provided with explosives such as picric acid; with perfumes and flavouring
materials like bitter-almond oil and vanillin; with a sweetening principle
like saccharin--compared with which the product of the sugar-cane is but
feeble; with dyes which tint the photographic film, and enable the most
delicate gradations of shade to be reproduced; with developers such as
hydroquinone and eikonogen; with disinfectants which contribute to the
healthiness of our towns; with potent medicines which rival the natural
alkaloids; and with stains which reveal the innermost structure of the
tissues of living things, or which bring to light the hidden source of
disease. Surely if ever a romance