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But, after making the gas, there was a thick, pitchy substance left from the coal, called coal-tar. It was only a trouble to the gas-makers, who had no use for it, and even threw it away, until some one, more thoughtful than the others, found out that water would not pass through it.
Another extremely sensitive indicator is the coal-tar dyestuff known as "Congo red"; the colour changes produced by it are exactly the inverse of those produced in the case of litmus, that is, it gives a blue colour with acids and a red colour with alkalis.
We have now before us a vast territory, namely, that of the b group of substantive colours, or, the largest proportion, indeed almost all of those prepared from coal-tar sources; Alizarin, also prepared from coal-tar, belongs to the adjective colours.
Separate the residuum in as many definite parts as possible, bottle and label, and keep accurate records as to process, weights, etc., so a reproduction of the experiment can at any time be made: Gelatine, 4 lbs.; asphalt, hard Cuban, 10 lbs.; coal-tar or pitch, 10 lbs.; wood-pitch, 10 lbs.; Syrian asphalt, 10 lbs.; bituminous coal, 10 lbs.; cane-sugar, 10 lbs.; glucose, 10 lbs.; dextrine, 10 lbs.; glycerine, 10 lbs.; tartaric acid, 5 lbs.; gum guiac, 5 lbs.; gum amber, 3 lbs.; gum tragacanth, 3 Lbs.; aniline red, 1 lb.; aniline oil, 1 lb.; crude anthracene, 5 lbs.; petroleum pitch, 10 lbs.; albumen from eggs, 2 lbs.; tar from passing chlorine through aniline oil, 2 lbs.; citric acid, 5 lbs.; sawdust of boxwood, 3 lbs.; starch, 5 lbs.; shellac, 3 lbs.; gum Arabic, 5 lbs.; castor oil, 5 lbs."
The raw material is a coal-tar naphtha called toluene or toluol, which is also the raw material for saccharin, a sweetening agent made from coal-tar. This artificial indigo is proving a formidable rival to the natural product. The lichens thus treated acquire gradually a deep purple colour, and form the products called "cudbear."
There was a heavy town-fog that afternoon, a smoke-mist, densest in the sanctuary of the temple. The people went about in it, busy and dirty, thickening their outside and inside linings of coal-tar, asphalt, sulphurous acid, oil of vitriol, and the other familiar things the men liked to breathe and to have upon their skins and garments and upon their wives and babies and sweethearts.
The waste of former generations is being utilised in this, and an instance is recorded in which tar, which was known to have been lying useless at the bottom of a canal for years, has been purchased by a gas engineer for distilling purposes. It has been estimated that about 590,000 tons of coal-tar are distilled annually.
If the blood pressure is high, such hypnotics will lower it, or if the heart is strong and the condition does not contraindicate it, aconite may be used in small doses, for a day or two, unless the fever is high and it seems advisable to use one of the coal-tar antipyretics, which reduce the blood tension and the heart activity. As stated above, pain must not be allowed.
Here the cost of transportation must be added to that of the coal, but the products are marketed near by instead of at a distance, as in the case of the bee-hive ovens. The most improved by-product ovens produce not only coke and gas, but coal-tar, pitch, ammonia, and creosoting oils, all extremely valuable and adding greatly to the value of the output of the ovens.
Some useful dyes are also obtained from it. It is obtained in quantities from coal-tar, that portion of the distillate known as the light oils being its immediate source. The tar oil is mixed with a solution of caustic soda, and the mixture is violently agitated.
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