THE ANILINE DYES. 
431 
attention here upon the oily and tarry products which condense 
in the hydraulic main and other condensers through which the 
gas must pass on its way from the retorts to the gasometers. 
This coal-tar , as it is called, on being put into a retort and 
distilled again yields different products at different tempera- 
tures, so that by a kind of fractional distillation we obtain two 
varieties of “ coal-oils ; ;; the heavy -oils which are heavier than 
water, and the light-oils which float upon water. The former 
are black and have a powerful odour ; they are extensively used 
to prevent the decay of railway sleepers, piles of bridges, 
palings, and other similar wooden constructions, on account of 
their powerfully antiseptic properties. In the hospitals of 
Paris I have seen them recently employed in the treatment of 
ulcerated wounds, for which purpose they are mixed with 
gypsum to solidify them a little, and to modify the intensity of 
their action. 
The light-oils constitute a thin combustible liquid known as 
coal-naphtha, which is distilled again to obtain Benzol. 
Lately, however, it has been attempted to distil the heavy-oils 
also, in iron or earthenware retorts, so as to decompose them 
into gas for illumination, and volatile oils rich in benzol. 
Analyses of several specimens of these heavy-oils which I 
made last year for the English Government show that they 
contain on an average : — 
Creosote and phenol 1*37 
Naphthaline 1976 
Brown oil 48*62 
Pitch 30*25 
100*00 
On being distilled they yield a large amount of naphthaline 
by decomposition , as well as other products, and if run into 
hot retorts they yield much carburetted hydrogen gas and 
Benzol » 
This Benzol, then, is the starting-point in the manufacture 
of the aniline dyes; for, the aniline that exists already 
formed in the coal-oils is in too small a quantity to admit of 
its being extracted with economy ; whereas benzol is easily pro- 
duced, is manufactured on a very extensive scale, and furnished 
to commerce at a low price. Without benzol we should have 
no aniline dyes, no beautiful mauve and magenta or rosaniline ; 
but as benzol can be produced wherever there is coal, there is 
no fear of our being deprived of these vivid colours for many 
centuries to come. 
It has been believed that the famous Tyrian purple of the 
ancients is extinct, — like the Dodo, &c. Such is not a fact. 
2 g 2 
