THE OLD METHOD OF CREAM-SEPARATION. 107 



the same as milk -fat or butter -fat. It is not correct to speak of the 

 percentage of cream or butter in milk, since cream and butter are not 

 milk constituents, but milk products. 



48. The Old Method of Cream - separation Cream - raising. - 

 According to the formula given in 6, it is easy to calculate the 

 acceleration which drives the fatty globules of the milk to the 

 surface (not taking into account any opposing forces) to be about 

 120 centimetres, or the eighth part of the acceleration of free-falling 

 bodies. The fatty globules in milk would, therefore, in the first 

 second of their movement, were it not for the friction due to their 

 movement, traverse 60 centimetres. Consequently, in layers of 

 milk not deeper than 60 centimetres the fat globules should be 

 collected on the surface in about a second's time. That this does not 

 actually take place, in point of fact, in cream -raising, is due to the 

 friction, which is exceedingly great in the case of the extremely 

 minute fatty globules. The ease with which single fatty globules 

 overcome resistance of different kinds is dependent solely on their 

 size. The large globules, of which some weigh 244 times more 

 than the smallest, overcome this resistance very easily, for they 

 come to the top in a deep milk layer very quickly, some of them 

 certainly in less than a minute. This is the case in warm fresh 

 milk. The smallest, on the other hand, are unable to overcome 

 this resistance and no longer exhibit independent motion, but follow 

 the milk-serum wherever it carries them. The rate at which the 

 globules tend to come to the surface depends directly on their 

 size. Were all the remaining constituents of milk in a state of 

 solution, the rising of the cream would take place with compara- 

 tive ease, since the fatty globules would only have to overcome the 

 internal friction and resistance which their motion entailed, and 

 the resistance offered by the currents caused by their movements 

 in the serum. But further opposition is experienced by them 

 through the fact that the caseous matter, and possibly also some 

 of the mineral salts of the milk, are not in a state of solution, but 

 are in a precipitated condition. We call the state of precipitation 

 perfect when it offers comparatively little resistance to the motion 

 of the fatty globules, and imperfect when it offers, on the other 

 hand, a large amount. Generally speaking, it may be said that the 

 state of precipitation of the caseous matter is most perfect in fresh 

 milk, and becomes gradually less so in the course of about three 



