It is worthy of note that the city of Lawrence, in Massachusetts, obtains the greater portion of its ice from a river which in its raw, unpurified condition was rejected for purposes of water-supply in consequence of the numerous and severe epidemics of typhoid fever which accompanied its use. Since the application of sand-filtration to this water, however, the death-rate from typhoid in this city, instead of being abnormally high, has fallen abnormally low, and this improvement is attributed to the excellent quality of the water supplied to the city, and has taken place despite the use which still continues of ice from the polluted river. The authorities consider the city's immunity from typhoid amply justifies their sanctioning the distribution of this river-ice, the freezing of the water having rendered it sufficiently pure to remove all danger to health from its consumption. So far we have been considering the effect on bacteria of freezing carried on under more or less natural conditions; but much interesting work of a more detailed character has been carried out with reference to the behaviour of particular varieties of micro-organisms when frozen under more or less artificial conditions. Thus Dr. Prudden froze various bacteria in water at temperatures ranging from -1° C. to -10° C., and he found that different varieties were very differently affected by this treatment; that, for example, a bacillus originally obtained from water, and introduced in such numbers as represented by 800,000 individuals being present in every twenty drops, after four days' freezing had entirely disappeared, not one having survived. On the other hand, similar experiments in which the typhoid bacillus was used resulted in the latter not only enduring a freezing of four days' duration, but emerging triumphant after it had been carried on for more than 103 days! In these experiments it should be borne in mind that, as the ice was frozen to a solid block or lump, there was no opportunity for the mechanical committal of the bacteria during freezing to the water beneath; all the bacteria present were imprisoned in the ice, and the fact that the typhoid bacteria were not destroyed by being frozen shows that they can withstand exposure to such low temperatures, although, as we have seen, the other variety of bacillus employed was destroyed. Dr. Prudden, however, discovered an ingenious method by which even typhoid bacilli were compelled to succumb when frozen. In the course of his investigations he found that bacteria which had offered the stoutest resistance under freezing were extremely sensitive to this treatment if the process was carried on intermittently, or, in order words, if the temperature surrounding them was alternately lowered and raised.