“Scillato decies si cor purgeris aceto
Anticipitesque tuum Samii Lucomonis acumen.”
[331]. In following the directions of the College the first pint is rejected, and this, according to Mr. Phillips (Remarks on the Pharmacopœia) contains a notable quantity of acid. Hence Distilled Vinegar can never be so strong as the Vinegar from which it is distilled.
[332]. Or it may be detected, in very minute quantities, by the elegant test lately employed by Dr. Marcet, and which I have frequently repeated in my Lectures with considerable satisfaction. It consists in adding a little sulphuric acid with a small quantity of muriate of soda, and then immersing a little gold leaf in the mixture, when after boiling it, if any nitric acid should have been present, the gold leaf will be dissolved.
[333]. It had been long known that by the destructive distillation of any kind of wood, an acid is obtained, which was formerly considered of a distinct and peculiar nature, and termed Acid Spirit of Wood, and afterwards Pyroligneous Acid. Glauber appears to have been the first chemist who was aware of its true nature, for he speaks of it as the “Vinegar of Wood.” It was however reserved for Fourcroy and Vauquelin to demonstrate its composition by experiment, and they have accordingly proved beyond doubt that it is merely the Acetic acid, contaminated with Empyreumatic oil and Bitumen. The address of modern chemists has at length enabled them to get rid of every trace of these latter ingredients, and to furnish an acid perfectly devoid of any foreign flavour. The crude pyroligneous acid, as it is first received, is rectified by a second distillation in a copper still, in the body of which about 20 gallons of viscid tarry matter are left from every 100. It has now become a transparent brown vinegar, having a considerable empyreuma; it is then redistilled and saturated with quick-lime, and the liquid acetate is evaporated to dryness and submitted to gentle torrefaction, in order to dissipate the empyreumatic matter, and lastly the calcareous salt is decomposed by sulphuric acid, when a pure, perfectly colourless, and grateful vinegar rises in distillation.
[334]. This instrument was invented by Messrs. Taylors for this particular purpose; the principle consists in forming a neutral salt with dry hydrate of lime and the acid to be examined, and then taking the specific gravity of the solution. Act 58. G. III. c. 65, § 8.
[335]. It may be necessary to state, that the Pharmaceutist should never purchase acetic acid of greater strength than that of 75° of the Acetometer, when it is intended for dilution, for although he might thus avoid the expense of carriage, the saving will be more than counterbalanced by the excessive duty levied upon acids above that standard. There is moreover a great loss in the preparation of strong acids, so that the manufacturer cannot afford to sell them at a price which is merely proportional to their strength. Acid of 75° is regularly kept by Messrs. Beaufoy for dilution, and if mixed with eleven parts of pure water is equivalent to the common distilled vinegar of the Pharmacopœia.
[336]. It ought to have been 1·048 of 55° Fah: but the error lies in the scale of Taylor’s Acetometer, which appears to be incorrect at this point.
[337]. I believe that no manufacturer, except Messrs. Beaufoy, makes an acid stronger than this; the College sample was obtained from that house.
[338]. The Reviewer of Mr. Phillip’s Translation of the Pharmacopœia, in the Royal Institution Journal for July, 1824, has fallen into an important error upon this subject, against which it may be necessary to caution the reader; he says, “the term ‘diluted acetic acid’ is properly enough applied to Distilled Vinegar, but the process of distillation might well have been rejected; for all medical purposes a dilute acid, composed of one part of the concentrated acid, contained in the Materia Medica, and four parts of water, is preferable. Of this mixture, or of distilled vinegar, the specific gravity should be 1·009, and 1000 grains should saturate 145 grains of Sub-carbonate of Soda.” The reviewer has mistaken the acid of sp. gr. 1·043 mentioned by Phillips, as the strongest he has met with, for the Pharmacopœia acid of sp. gr. 1·046; for, should he dilute the latter with only four times its weight of water, he would produce a compound containing 5·686 per cent. of real acid, or one considerably stronger than the strongest malt vinegar, and twice the strength of distilled vinegar. The reviewer takes this occasion to indulge his favourite passion for abusing the Pharmacopœia, and he asks with an air of sarcasm, Where was Dr. Paris during the late revision? I answer—engaged in the discharge of my duty as a humble member of the Committee, and I can assure him that nothing which he has yet urged has convinced me that I have failed in its fulfilment, or erred in its execution:—but it is now my turn to enquire, and I do so with perfect good humour, where the reviewer could have been when he composed the above passage? that he was not at home, is I think sufficiently evident from the statement which I have just offered.