206 BACTERIA 



is, however, a difficult matter to determine what amount of 

 boric acid is injurious to health, for this differs widely in 

 different persons. It has been laid down by one authority 

 that even so small an amount as one-tenth per cent, might 

 have inconvenient results, owing to its cumulative effect. 

 Formaldehyde is without doubt an excellent antiseptic, and 

 the more its efficacy becomes known so much the more 

 probably will it be used. The salicylates, which are mild 

 antiseptics, have long been used as preservatives. These 

 substances, then, can be added to milk in quantities not 

 recognisable to the taste (salicylic acid about .75 grain, and 

 boracic acid .4 grain, to the litre of milk). They will ma- 

 terially increase the time that milk will remain sweet, they 

 will prevent a number of micro-organisms living in the milk, 

 and will inhibit multiplication of others. 1 Secondly, it is 

 possible very perceptibly to remove the infectivity of milk 

 by filtration and temperature variations. 



Filtration has been practised for some time by the Copen- 

 hagen Dairy Company and by Bolle, of Berlin. The filters 

 used consist of large cylindrical vessels divided by horizontal 

 perforated diaphragms into five superposed compartments, 

 of which the middle three are filled with fine sand of three 

 sizes. At the bottom is the coarsest sand, and at the top 

 the finest. The milk enters the lowest compartment by a 

 pipe under gravitation pressure, and is forced upwards, and 

 finally is run off into an iced cooler, and from that into 

 the distribution cans. By this means the number of bac- 

 teria is reduced to one-third. The difficulty of drying and 

 sterilising enough sand to admit a large turnover of milk is 

 a serious one. This, in conjunction with the belief that 



1 S. Rideal and A. G. R. Foulerton conclude, from a series of experiments, 

 that boric acid (1-2000) and formaldehyde (1-50,000) are effective preserva- 

 tives for milk for a period of twenty-four hours, and that these quantities have 

 no appreciable effect upon digestion or the digestibility of foods preserved by 

 them {Public Health, May, 1899, pp. 554-568). 



