~"Sanitary" or "certified" milk supplies.~ In a number of the larger cities, the attempt has been made to improve the quality of the milk supplies by the installation of dairies in which is produced an especially high grade of milk. Frequently the inspection of the dairy as well as the examination of the milk at stated intervals is under the control of milk commissions or medical societies and as it is customary to distribute the certificate of the examining board with the product, such milks are frequently known as "certified." In such dairies the tuberculin test is used at regular intervals, and the herd inspected frequently by competent veterinarians. The methods of control inaugurated as to clean milking and subsequent handling are such as to insure the diminution of the bacteria to the lowest possible point. The bacterial limit set by the Pediatric Society of Philadelphia is 10,000 organisms per cc. Often it is possible to improve very materially on this standard and not infrequently is the supply produced where it contains only a few thousand organisms per cc. Where such a degree of care is exercised, naturally a considerably higher price must be paid for the product,[126] and it should be remembered that the development of such a system is only possible in relatively large centers where the dealer can cater to a selected high-class trade. Moreover, it should also be borne in mind that such a method of control is only feasible in dairies that are under individual control. The impossibility of exercising adequate control with reference to the milking process and the care which should be given the milk immediately thereafter, when the same is produced on different farms under various auspices is evident. PRESERVATION OF MILK SUPPLIES. While much can be done to improve the quality of milk supplies by excluding a large proportion of the bacteria which normally gain access to the milk, and preventing the rapid growth of those that do find their way therein, yet for general municipal purposes, any practical method of preservation[127] that is applicable on a commercial scale must rest largely upon the destruction of bacteria that are present in the milk. The two possible methods by which bacteria can be destroyed after they have once gained access is (1) by the use of chemical preservatives; (2) by the aid of physical methods.