The Romance of Modern Chemistry

VALUABLE SUBSTANCES indiarubber or making varnish, and some was employed for illuminating purposes. A patent flare lamp for the combustion of this naphtha was invented sixty years ago, and is still part of the regular equipment of a coster’s barrow. The thorough utilisation of coal tar was not possible until chemists had made a complete study of its con- stituents. This was carried out to a large extent by the middle of last century, and one result of these investi- gations was to demonstrate the presence in coal tar of the following important compounds:—benzene, toluene, phenol or carbolic acid, naphthalene, and anthracene. All of these, except phenol, are compounds of carbon and hydrogen, that is, hydrocarbons, and their importance arises from the fact that they are the starting-points for the manufacture of the aniline dyes and other synthetic products of that kind. The actual proportion of these five compounds in coal tar is not great—as a rule, perhaps less than 12 per cent., but they are the constituents which chiefly concern us here. They are extracted from coal tar by the process of distillation. Some are much more volatile than others, and when the tar is boiled these distil over first, are con- densed, and so are separated from the less volatile con- stituents. The temperature of the tar in the boiler is continuously raised, and the process of separating a more volatile from a less volatile part is repeated. In this way the tar distiller obtains a number of portions or “ fractions,” and has finally left in his boiler a quantity of pitch, amounting to about 60 per cent, of the original tar. The various fractions in which the distillate is collected are known as “ first runnings,” “ light oil,11 “ carbolic oil,” “ creosote oil,” and “ anthra- 284