In 1859, a demand arose for a purer acid. From experiments instituted by Dr. Calvert, it was found that the best mode of preparation was not by treating light or heavy oils of tar with con centrated alkalies ; but, on the contrary, by treating the impure benzols or naphthas of commerce —distillates from the tar oils—with weak alkaline solutions. By this means, was produced a blackish fluid, a little heavier than water (sp. gr. 1 .060), and containing 50 per cent. of real carbolic acid, which latter was separated in part by careful distillation. This acid continued in use for colour manufacturing till 1861, when aniline colours of such fineness and brilliancy were produced, that, in order to keep pace with them, it became necessary to still further improve the quality of the carbolic acid. After some trials, white detached crystals of the acid, melting at about 29° (85° F.), were obtained. In 1863, this relative purity was again found to be insufficient, and further efforts to increase it resulted in the production, on a commercial scale, of Laurent's " phenylio alcohol," a substance melting at 35° (95° F.), and boiling at 186° (367° F.). Repeated attempts to draw the attention of the medical profession to the remarkable therapeutic qualities of this acid were then made ; but the tarry and sulphuretted odours which it still possessed were serious obstacles in the way of its application. Dr. Calvert soon succeeded, however, in removing these objectionable features, and was able, in 1861, to manufacture an acid quite free from sulphuretted smell. Still he did not stop his researches ; but, two years later, discovered a process which enabled him to show an acid completely deprived of all disagreeable odour and tarry flavour, and as pure, though extracted from tar, as if it had been produced by the help of the reactions noticed by Wurtz and Heinle, based upon the direct transformation of benzol into carbolic acid, or by the well known changes by which it may be obtained from salicylic or benzoic acids. This new phenylic alcohol, or carbolic acid, was, however, in some respects distinguished from Laurent's. Thus, it was soluble in 12/ parts of water instead of 33 ; was fusible at 42° (103° F.), instead of at 35° (95° F.); and boiled at 182° (360° F.), instead of at 187° (368° F.). Nevertheless it gave, like Laurent's, the blue colour described by Berthelot as being produced on mixing ammonia with it and adding a small quantity of hypochlorite to the solution, the same effect being produced by exposing to hydrochloric acid vapours a chip of deal that has been soaked in this pure carbolic acid. It was supposed that, as Laurent's acid had constant boiling and crystallization points, it was a pure and definite substance ; but the production of this pure acid proved it to be nothing of the kind, Laurent's article being only a combination of pure carbolic acid with a liquid homologue; for when a certain proportion of water is added to Laurent's acid, and the mixture is exposed to a temperature of 4° (39° F.), it deposits large octahedrons of a crystalline substance, which is a hydrate of carbolic or phenylic alcohol, that is to say, carbolic acid combined with an equivalent of water of crystallization. This fo,ct is highly interesting from a chemico-theoretical point of view, for it exhibits the only example known of an alcohol which, combining with water, forme a crystalline hydrate. By removing from this hydrate the equivalent of water, es well aa the traces of sulphuretted compounds, and coal-tar baaes, which it contains, carbolio or phenic acid is obtained iu its pureat state.
Present Method of Manufacture.—To procure crude carbolic acid, the coal-tar is distilled in a still much resembling in all reapects that used for the diatillation of the acid (to be ahown presently)/ only of about twenty times aa great a capacity. The distillate from the coal-tar is collected in various portiona, as already indicated, the largeat proportion of the carbolic acid being generally prE sent in the " light oils," a black apirit having,' a powerful, unpleaaant, tarry odour. Thia tar oil is mixed with s, caustic soda solution at 12° Tw., made from cream °audio soda, aud ia put into a barrel shaped boiler fitted inaide with arms, which are made to revolve on a spindle, and thus thoroughly agitate the mix ture. Tbe result of this process is that the cauatic soda dissolvea out the whole of the carbolic acid, while the separated and undiaaolved oily matters, known as creoaote oil, float on the surface of the solution when it is allowed to settle. The alkaline solution ia then run off from the supernatant
oils, and is treated with brown oil of vitriol (aulpburio acid at about 140°-150° Tw.), in juat sufficient amount to completely neutralize the aoda, without being in exceas. This acid forma a aalt with the soda—sulphate of soda—which sinks, while the carbolic acid rises to the aurfaco. Generally, the sulphate of soda is allowed to settle out twice, eo as to leave the carbolio acid aa free from it as possible. At thia stage, it is known as " crude " carbolic acid, and, though considerably purified, it still retains a deop black colour, and an unpleasant odour. The pro duct ia received, in thia state, from the tar diatillera. The proportion borne by thia a,cid to the amount of tar treated in its production is about 1-3 per cent. The impure or crude carbolic acid consiats of carbolic and eresylic acida, and their homologues, together with a variety of im purities. It is next subjected to a procesa of fractional distillation, which separates the carbonaceous matters and the water, the latter amounting to about 15 per cent. of the whole. This operation is conducted in the apparatua ahown in Fig. 491 ; A is a circular still of wrought iron, 4 ft. in diameter, 6 ft. high, and provided with a dished bottom ; it is set iu brickwork (as ahown in Fig. 492), with a double series of flues, one to heat the upper portion of the aidea, viz. the apace included between e and f ; the other, to heat the aides from f downwarda to g ; above the level of e, the still is furnished with a manhole a; on to the top of the still, a cast-iron head and arrn B ia bolted, as shown at b b, the head B being fitted with a flange for that purpose ; at c, anotber flange is cast on the arm B, for the reception of a flange c of the leaden condenaing worm C ; thia condenaing worm is of 2 in. bore at the commencement, diminishing to in. at the outlet d.
The distillate which escapes at d is collected in metallic coolers, about 22 in.
high, bolding about 12 gallons, and of the ahape indicated in Fig. 493. They are placed in trougha, and surrounded by a refrigerating mixture, produced by some cloling apparatus, aucla as Kirk's, or Siddeley and Mackay's, refrigeratiog machine. The laat is shown in Fig. 494 ; A is the refrigerator ; B, tha vacuum purnp ; C, the condenser ; D, the ether meter ; E E, the water pumps ; F, the hand pump ; G, the band-pump condenser ; I, the 2 x steam engine. The refrigerator A, a copper tubular vessel, is charged with the requisite supply of liquid ether, which, by the action of the vacuum pump B, is evaporated, drawn away in the form of vapour, and passed into the copper tubular condenser C ; here, under slight pressure, and by the aid of a stream of water, it is again reduced to its liquid state, and is thence returned through the ether meter D to the refrigerator A, to be re-evaporated. Thus the same ether is used continuously, with inappreciable loss. The ether meter D regulates the flow of the liquid ether to the refrigerator, rendering the machine self-acting. To utilize the cold produced by the evaporation of the ether, an uncongealable liquid, auch as very strong brine, or a solution of calcic chloride, is forced by the pump E through the tubes of the refrigerator A, parting with its heat to the ether vapour on its passage, and leaving that vessel at a temperature of — 10° to — 7° (14° to 20° F.)—thus many degrees below freezing point—to be uaed in the freezing tanks, where the coolers represented in Fig. 493 are placed. The carbolic acid requires a long time for cooling; but solidifies finally in the form of thick acicular crystals of irapure acid, which have lost their carbonaceous impurities so far as to be only slightly drab-coloured; and, though strongly odoriferous, their smell is by no means unpleasant. After removal from the freezing tank, the coolers are drained, by withdrawing the cork that is inserted in the aperture at the bottom. The liquid present escapes, and leaves a perfectly white acid, of indefinite crystalline form, fusing at about 29° (85° F.), but still possessing a strong odour. This ia known as Calvert's " No. 3." acid.