The tar oil is mixed with a solution of caustic soda, and the mixture is violently agitated.  This results in the caustic soda dissolving out the carbolic acid, whilst the undissolved oils collect upon the surface, allowing the alkaline solution to be drawn from beneath.  The soda in the solution is then neutralised by the addition of a suitable quantity of sulphuric acid, and the salt so formed sinks while the carbolic acid rises to the surface.

Purification of the product is afterwards carried out by a process of fractional distillation.  There are various other methods of preparing carbolic acid.

Carbolic acid is known chemically as C_{6}H_{5}(HO).  When pure it appears as colourless needle-like crystals, and is exceedingly poisonous.  It has been used with marked success in staying the course of disease, such as cholera and cattle plague.  It is of a very volatile nature, and its efficacy lies in its power of destroying germs as they float in the atmosphere.  Modern science tells us that all diseases have their origin in certain germs which are everywhere present and which seek only a suitable nidus in which to propagate and flourish.  Unlike mere deodorisers which simply remove noxious gases or odours; unlike disinfectants which prevent the spread of infection, carbolic acid strikes at the very root and origin of disease by oxidising and consuming the germs which breed it.  So powerful is it that one part in five thousand parts of flour paste, blood, &c., will for months prevent fermentation and putrefaction, whilst a little of its vapour in the atmosphere will preserve meat, as well as prevent it from becoming fly-blown.  Although it has, in certain impure states, a slightly disagreeable odour, this is never such as to be in any way harmful, whilst on the other hand it is said to act as a tonic to those connected with its preparation and use.

The new artificial colouring matters which are continually being brought into the market, testify to the fact that, even with the many beautiful tints and hues which have been discovered, finality and perfection have not yet been reached.  A good deal of popular prejudice has arisen against certain aniline dyes on account of their inferiority to many of the old dye-stuffs in respect to their fastness, but in recent years the manufacture of many which were under this disadvantage of looseness of dye, has entirely ceased, whilst others have been introduced which are quite as fast, and sometimes even faster than the natural dyes.

It is convenient to express the constituents of coal-tar, and the distillates of those constituents, in the form of a genealogical chart, and thus, by way of conclusion, summarise the results which we have noticed.