The rather common belief that freezing purifies water is incorrect. It is said, that the greater part of the ice supply of New York City (three millions of tons) is gathered from the Hudson River between Albany and Poughkeepsie, most being drawn within thirty miles of the former city, and therefore liable to be polluted with sewage. The average number of bacteria in one c.c. of ordinary ice is stated to approximate 400, but Hudson River ice has been found to contain nearly 2000 bacteria per c.c.[132] The number of bacteria in one c.c. of snow is usually about 9000; Hudson River snow-ice contains 20,000 per c.c.; and, although the great majority of these organisms are perfectly harmless, cases are on record where epidemics (as of gastro-enteritis) have been directly traced to the use of impure ice. FOOTNOTES: [119] This test presupposes the existence in the water of the substances necessary for the support of vegetable growth. [120] ‘Water Analysis.’ [121] Jour. Lond. Chem. Soc., xviii. p. 117. [122] Ibid., xxxv. p. 67. [123] Sixth Annual Report, Rivers Pollution Commission, “Blue Book.” [124] Jour. Lond. Chem. Soc. 1867, xx. p. 445. [125] Phil. Mag., xxx. p. 426. [126] Ber. der Deutsch. Chem. Gesell. xii. p. 427. [127] Chem. Soc. Journ., March, 1879. [128] Pharm. Zeit., 1885, No. 76. [129] ‘Annual Report of the National Board of Health,’ 1882, p. 207. [130] Jour. Soc. Chem. Indus., Dec. 1885. [131] Jour. Am. Chem. Soc., viii. p. 6. [132] See paper read by Dr. T. M. Prudden before the New York Academy of Medicine, March 18th, 1887. VINEGAR. Vinegar is a dilute aqueous solution of acetic acid, containing inconsiderable proportions of alcohol, aldehyde, acetic ether, and extractive matters, which, to some extent, impart a characteristic flavour and aroma. The process most frequently involved in the preparation of vinegar is known as the acetous fermentation, and may be induced in various saccharine juices and infusions, such as those of apples, wine, malted grain, etc., when, in presence of a ferment, they are exposed to the action of the air, at a temperature between 24° and 32°. In the oxidation of alcohol, an intermediate compound (aldehyde) is at first formed, which, by the continued action of oxygen, is ultimately converted into acetic acid. A dilute solution of alcohol is, however, not oxidised to acetic acid by simple exposure to the air; it is usually necessary that a peculiar microscopic plant (_mycoderma aceti_) should be present. This fungus includes two varieties, viz., minute globules (_micrococci_) and rod-like forms (_bacilli_) varying in size; and is often developed in old casks that have been long employed for making vinegar. It constitutes a gelatinous mass (“mother of vinegar”) having the appearance of glue that has been soaked in cold water; the surface quickly becomes coated with a bluish mould (_Penicillium glaucum_).