^86 Transactions of the Canadian Institute. [Vol. VII. 



reported in 1893 that the bacterial content of milk was reduced 48 per 

 cent, by sand filtration ; and at the meeting of the German Dairy- 

 Association in 1893, he strongly recommended the use of sand filters for 

 removing dirt and germs from milk. The use of sand filters was, 

 however, questioned by other members. 



Backhaus,5s whilst giving no numerical data, reports that these 

 filters have no effect in reducing the number of bacteria in the filtered 

 milk. The mechanical separation is good, all coarse particles, such as 

 hair, straw, manure, etc., are arrested ; but the bacteria are washed out of 

 the manure, and the milk contains more bacteria than before filtration. 



In 1899, Dunbar and Kister^^ made an exhaustive study of the 

 working of this class of filter. In twenty-two analyses of raw and 

 filtered milk there were in seventeen cases more bacteria present after 

 filtration, and in four cases fewer bacteria. A few examples of their 

 results will suffice : 



Raw Milk. Filtered Milk. 



80,000 per c.c. 60,000 per c.c. 



793,000 " 44,100 " 



95,000 " 49400 



819,000 " 94,000 " 



Average 446,700 " Average 61,800 " 



In these cases filtration diminished the bacterial content of the milk. 



Of the seventeen other samples, the bacterial content after filtration 

 was increased, thus : — 



Raw Milk. Filtered Milk. 



350,000 per c.c. 600,000 per c.c. 



650,000 " 950,000 " 



650,000 " 1,260,000 



320,000 " 620,000 " 



3,900,000 " 14,300,000 " 



The average of the seventeen analyses was : — 



Raw Milk. After Filtration through Gravel. 



1,300,000 per c.c. 5,567,000 per c.c. 



Cleaning Milk by Centrifugal Force. 



Clarified milk, {see Fig. 11), or milk that has been passed through 

 a separator, has been recently quite extensively advertised. The effect 



