Pasteur regards acetification as a product of the development of the _mycoderma aceti_, _i.e._, as a physiological fermentation--but it appears probable that the process is rather one of oxidation, and that the fungus accelerates the change by condensing the oxygen upon its surface and delivering it to the alcohol, possibly in the form of ozone. Indeed, the process of vinegar making may take place in the entire absence of the _mycoderma_, as when spongy platinum is brought into contact with alcoholic solutions; and Buchner has examined shavings which had been used in a vinegar factory for over twenty-five years, and found them to be absolutely free from the fungoid plant. In the United States, the best known and most esteemed kind of vinegar is that obtained by the acetification of apple cider; but by far the largest quantity is manufactured from alcohol and spirituous liquors. Cider vinegar is free from aldehyde but contains malic acid. The usual source of vinegar in Great Britain is a wort prepared from mixtures of malt with other grain; while, in Continental Europe, inferior sorts of new wine (especially white wine) are extensively employed for its production. Malt vinegar possesses a brown colour and a density ranging from 1·006 to 1·019; that known as proof vinegar contains from 4·6 to 5 per cent. of acetic acid. In Great Britain the manufacturer is allowed by law to add 0·1 per cent. of sulphuric acid to vinegar, on account of its supposed preservative action, and, although the practice is now known to be unnecessary, it is still sometimes resorted to. The specific gravity of wine vinegar varies from 1·014 to 1·022. 100 c.c. should neutralise from 0·6 to 0·7 grains of sodium carbonate, and the solids obtained upon evaporation to dryness should approximate two per cent. According to the United States Pharmacopœia, one fluid ounce of vinegar should require for saturation not less than 35 grains of potassium bicarbonate. In 500 samples of imported wine and malt vinegar tested by the author, the minimum and maximum strength ranged from 3 to 10·6 per cent. of acetic acid, the specific gravity from 1·0074 to 1·0150, and the number of grains of potassium bicarbonate required to neutralise one troy ounce from 22 to 84. Of 273 samples of vinegar tested in 1884 by the Massachusetts State Board of Health, 52 were above the then legal standard of 5 per cent. of acetic acid, and 221 below; 109 of the latter contained more than 4 per cent.; the strongest sample showed 8·86 per cent., and the weakest contained but 0·66 per cent. of acetic acid. In the year 1885, 114 samples were examined, of which 45 were above and 69 below the standard of 4½ per cent. acetic acid.