146 THE BACTERIA CONTENT OF MILK 



separated into two layers, the outside being separated milk and the 

 inside separated cream. As the revolutions continue, there comes 

 to be a sediment on the wall of the bowl consisting of what is 

 sometimes termed the " slime." By means of a simple contrivance 

 the layer of milk is conducted to an exit tube on the one side 

 of the machine, and the layer of cream to a separate exit tube 

 on the other side. The whole milk, therefore, which was poured in 

 at the top of the machine has, by centrifugal action, become 

 separated into three parts — the cream, the milk, and the " slime " 

 left as a sediment or film on the walls of the bowl, most of 

 which eventually falls to the bottom of the bowl. We have made 

 many examinations of these three constituent parts, with the 

 result that the majority of the bacteria contained in the whole 

 milk are found eventually in the "slime" or sediment of the bowl. 

 Such bacteria have been driven out to the periphery by centri- 

 fugalisation, first passing from the inner layer (the cream) through 

 the second layer (the separated milk) into the third layer (the 

 " slime "). Hence it comes about that this mechanical separation 

 results in " filtering " the milk of many of its bacteria, and driving 

 them into the sediment, which is discarded. The bowl sediment, 

 therefore, contains the majority of the organisms, the separated 

 milk being next in order, and the separated cream containing 

 fewest organisms. These results are illustrated on Plates i8 and 19. 

 Separated cream contains comparatively few organisms derived 

 from the air unless some period has elapsed between the drawing 

 of the milk and its separation. In this respect it is similar to set 

 milk. Set cream and whole milk, on the contrary, always show a 

 number of air and chromogenic organisms, which begin to dis- 

 appear when the tide of lactic acid organisms commences. 



If we compare, by means of the quantities of bacteria present, 

 the set and separated constituent parts of milk, we find they come 

 out in the following order, viz.: (i) the bowl sediment contains 

 most organisms ; (2) next to it, but comparatively similar to it, the 

 set cream ; (3) the separated milk ; (4) the separated cream ; (5) the 

 whole milk ; and lastly (6), the set milk. 



It may be assumed that this centrifugal action of the separation 

 upon bacteria in milk acts in a uniform manner upon all organisms, 

 controlled chiefly, in all probability, by two simple conditions, 

 namely, the entanglement of the organism amongst the fat cells, 

 and the weight and body of the organism itself. It should be said 

 in passing that the quantity of slime is affected by the acidity of the 

 milk and other conditions. In a general way, then, the slime or 



