under that article. See _Acetum_. _Sulphurous Acid_ may be recognised by drawing a little of the vapour into the lungs. The presence of _Nitric Acid_ may be discovered by saturating the suspected sample with pure potass, evaporating to dryness, and then treating the product with a highly concentrated alcohol, the acetate of potass will be thus dissolved, but as it exerts no action on the _Nitrate_ it will be found in the residuum, and may be recognised by its deflagration, when thrown upon burning charcoal;[332] _Copper_ may be detected by the acid assuming a blue colour, when supersaturated with ammonia; and _Lead_, by a solution of sulphuretted hydrogen, producing a dark coloured precipitate. _Tin_ however is the metal with which distilled vinegar is more usually contaminated, for no vegetable acid will act upon lead while any tin is present in the mixture, since the latter, being more oxidable than the former, is exclusively dissolved. ACIDUM ACETICUM FORTIUS. L. (_Acidum Aceticum._) (_e Ligno Destillatum._) _vulgo, Pyroligneous Acid._ The acetic acid from wood has been very generally introduced to supersede the use of distilled vinegar for the purposes of Medicine and the Arts.[333] It is at length found to be capable of such complete separation from all foreign matter as to afford a perfectly pure acetic acid, invariable in its acidifying power, and immutable in its chemical properties. In justice to the skill and industry of Messrs. Beaufoy and Co. of South Lambeth, I beg to state that I have examined various specimens of this acid from their manufactory, and that I find it free from those impurities which have hitherto constituted an insuperable objection to its introduction into the Materia Medica. The purified _Pyroligneous_ acid, manufactured by this company, and sold under the name of “_Improved Distilled Vinegar_,” is perfectly free from any unpleasant taste, as well as colour and sediment; and it forms a limpid and colourless solution with ammonia. The common distilled vinegar of the shops varies essentially in strength as well as purity, differing in acidifying power from 30 to 40 per cent. in value: it is sometimes 7 degrees, and at others less than 5, by the Revenue Acetometer;[334] and hence has arisen the difficulty of procuring an uniform article for medical application, a difficulty which the introduction of the _pyroligneous_ acid seems calculated to overcome, as it may be procured from the manufacturers of any degree of concentration,[335] from 6 degrees of the Acetometer, or 2·826 per cent. of real acetic acid to 130 degrees, or 61·49 per cent. of acid; and even of still higher strength if required; their common, or _Proof_ acid is about equivalent in strength to that of the best Malt Vinegar, of which 100 grains will saturate 14½ grains of crystallized _Sub-carbonate of Soda_, and consequently contains 4·73 per cent.