In dealing with drinking water on a large scale, such as the disinfection of wells and tanks, we have several available methods, for one or the other of which the materials are almost everywhere available. The most valuable of these agents is undoubtedly the permanganate of potash, and the suggestion of its use for the disinfection of drinking water from the germs of cholera is undoubtedly due to Mr. Hankin, our official bacteriologist at Agra. No doubt more than one medical officer had previously made experiments with this chemical for the purification of drinking water, but the credit of definitely proposing its use on a large scale in cholera epidemics, and of proving that it is lethal to the cholera germ, even when greatly diluted, is undoubtedly due to him alone. The enormous practical importance of the discovery has, however, been but slowly realised, though its capabilities were put to the test of practical application by the writer immediately after Mr. Hankin published his suggestion, with the result that a severe epidemic in a town of over 10,000 inhabitants was brought to an abrupt termination within three or four days. The method of disinfection of wells by means of this chemical has already been described on page 47, _et seq._ The addition of a few ounces of common sulphuric acid increases, I believe, the lethal effects of the permanganate on the cholera germ, but is certainly not necessary, and I have no personal experience in its employment, as the taste imparted is somewhat persistent; and in dealing with suspicious races such as those inhabiting our Indian possessions, it is desirable that all obvious change in the taste or appearance of the water should pass off as soon as possible. The alternative agents are alum and quicklime, either of which is very fairly effectual, though by no means as trustworthy as permanganate. Their great advantage lies in the fact that they are obtainable almost everywhere, and that being familiar articles of daily life their use is less likely to give rise to misunderstandings in dealing with ignorant and suspicious populations, who regard with mistrust the treatment to their wells with a chemical so strange and striking as permanganate must appear to them. Nearly all races are, however, familiar with the cleansing powers of lime, and in the case of Indians, the wonderful powers of alum in clearing turbid water is a bit of household knowledge familiar to everyone. A pound or two of alum, or half a hundredweight of lime, are required for each well. If alum is employed, it should be roughly powdered.