Neglecting this "No. 3 " acid for the present, it will he well first to follow the mother-liquor which .bas been drained from it. This is placed in a still, similar in all respects to that already described; 300 gallons of the liquid, of which t4 very variable proportion will be water, are redistilled, until there only remains in the bottom of tbe still about 30 gallons of a pitchy residue, which is run off, while hot, in a fused state. The distillate is conducted through a leaden worm surrounded by water, for condensation, and thence into coolers. In thia product, there is present a certain amount of crystallizable carbolic acid, as well as more or less of cresylic acid. This latter, which is soluble in about 80 parts of water, and whose boiling point is 190°-200° (374°-392° F.), is a constituent of all impure commercial carbolic acids, and is extensively used as a disinfectant, being frequently sold, for this purpose, under the name of " crude" or " liquid" carbolic acid. It may here be mentioned that there has grown up a practice of passing off, under this narae, mixtures of tar oils, containing usually only a small percentage of carbolic acid, and sometimes even none at all, their general appearance and odour preventing their ready distinction, by the unscientific public, from the genuine acid. Now the value of the liquid for disinfecting purposes depends upon the quantity of carbolic and cresylic acids present in it, and upon its freedom from tar oils, for these, even if containing a small percentage of carbolic acid, aro them selves comparatively valueless for disinfection, and, being insoluble in water, hinder the solubility of the carbolic acid. No liquid carbolic acid should be purchased, therefore, without a guarantee from the vendor as to its composition.
A ready method of testing liquid carbolic acid, to ascertain its genuineness, is to measure off a given volume in a graduated glass, and then to add to it twice its volume of a pure caustic soda solution of 14° Tw. at 15° (60° F.). As carbolic and cresylic acids are both soluble in this solution, the " liquid" acid ahould, if genuine, entirely dissolve, on shaking the mixture well together. The test for the crystallized acid ie its fusing point ; and, for the liquid acid, its solubility.
F. C. Calvert and Co. do not eell this liquid acid iu its impure state; tiny subject it to another distillation, and remove all trsces of sulphuretted hydrogen, producing a very slightly tinted, clear liquid, having a much less rank odour. This they distinguish ae " No. 5 " acid, which they sell in bulk (caske, &c.) ; 6s. per gallon is the retail price. By recliatilling this "No. 5," they get a still purer aoid, called " White No. 5," which is the ordinary liquid acid sent out by them in bottles. This, as well as the preceding quality, is guaranteed to contain not less than , 85 per cent. of carbolic and cresylic acids, and to be free from tar oils and sulphuretted hydrogen.
To return to the " No. 3 " acid, fusing at 29° (85° F.). This is redistilled in the same kind of still; but the worm for condensing the gases ae they are evaporated has now to be made of zinc, instead of lead, as the latter metal would colour the product. The distillate is run into coolers of tin or zinc, where it is allowed to cool in the air down to 32° (90° F.). The coolers are then
pierced, to dmin off the liquid for reworking, leaving a mass of pure white cryetals, fusing at 35° (95° F.), and correeponding with the acid produced hy Laurent. This is Calvert's " No. 1 Com mercial " acid, used chiefly for the manufacture of the varioue carbolic acid colours. It is frequently aold as purporting to be " B. P." quality ; but to obtain the real B. P. acid, it ia necessary to remove from this commercial acid all traces of sulphuretled compounds and coal-tar bases, and to rectify it in ordinary gime ewan-neek retorts of about 4 gallons capacity, and having long glass tubes ae condensere for the dietillate. Here the acid must be treated aud evaporated almost to dryness. The cooled distillate has the same fueing point as the acid whence it is derived. It is soluble in 20 pat te of water, and should be used exclusively for surgical applications. It forms Calvert's " No. 2 Medicinal " carbolic acid, and is the only one which fulfils all the conditions of the Mitish Pharmacopoeia, drawn up by the late Dr. Crace Calvert.
In order to produce a more perfectly pure carbolic acid, advantage is taken of Lowe ttnd Gill's patented invention (1874), whose object is to effect and facilitate the sepaiation of carbolic aoid from the cresylic and other liquid tar acids contained in mixtures of these products, which, when dehydrated by distillation in the ueual manner, are crystallizable at 17°-35° (623-95° F.). The following method is adopted :--It is first ascertained whether the acids are partially or wholly hydrated ; should they be in a dehydrated sts,te, they must be hydrated by the addition of water to the amount of 5-30 per cent. The hydrated acids aro then placed in suitable vessels surrounded by a cooling mixture, or circulating fluid, of a temperature sufficiently low to effect the separation of more or less hydrat,ed carbolic acid crystals, say — 9° to 13° (15° t,o 56° F.). This refrigerating process being complete, the mother-liquors are drained or otherwise separated from the crystals, and are rectified in the msnner already described, to bring them within the limits of the crystallizable temperatures for re-treatment.
The crystals may be purified from all traces of the mother-liquors, by re-crystallization, either by partial fusion, or by solution in water and subsequent refrigeration of the water solution to a temperature of about 1° (33° F.). The crystals are then dehydrated by fractional distillation, and thus is produced a carbolic acid crystallizing at 38°-421° (100°-108°•5 F.), and boiling, with thermometer in liquor, at 181°-182° (358°-360° F.) under an atmospheric pressure of 29.26 in. of mercury. Calvert and Co. drain away the mother-liquor from this acid, and subject the latter to a special treatment, rendering it perfectly free from traces of sulphuretted compounds and coal-tar bases, then to a further rectification in glass, producing a chemically pure acid, fusing at 42°•2 (107°•9 F.), boiling at 182° (360° F.), and soluble in 121 parts of water. This pure acid is composed of acicular crystals, and is entirely free from tarry taste and odour; it is known as "No. 1 Medicinal" acid, and is used nearly exclusively for internal administration. It is sold in bottles of 1 oz. and upwards, the retail price being 10s. per lb.