(HAP. v. TUBERCULOUS MILK. 305 



producing the disease (tuberculosis or consumption) in young or weakly 

 subjects consuming it. The chief difficulty in determining whether 

 the milk of any particular cow or cows is dangerous lies in the inability 

 of the veterinary surgeon to say whether there are any tubercular 

 deposits in the udder. Milk may contain these organisms and even a 

 skilled bacteriologist fail to find them ; their absence in the few drops 

 which he examines is no guarantee that they may not exist. Recent 

 experiments in the United States have demonstrated that where 

 tuberculous cows showed no signs of the disease in the udder their milk 

 nevertheless proved infective to rabbits and guinea-pigs fed with it. 

 The results of feeding experiments all tend to prove that the milk 

 from tuberculous cows, if given to animals in the uncooked state, 

 possesses a very much higher infective power than the flesh. 



Butter. Milk consists mainly of three component parts, the butyra- 

 ceous, or oily or fatty substance of which butter is composed ; caseous 

 matter, from which cheese is formed; and the serum, or whey. The 

 comparative value of different dairies, and of different cows in each 

 dairy, depends not only on the quantity of milk itself, but also on the 

 quantity of butter or casein it contains. The ingredients named 

 differ materially in specific gravity or weight, and to separate them is 

 the chief object of the dairy. The cream is the lightest, next in specific 

 gravity is the whey, and the curd is the heaviest. The manufacture of 

 butter involves the separation of the butyraceous part, and this is a 

 mere question of gravity. The milk is left undisturbed, and thus the 

 lighter portion mechanically quits the heavier one, and floats on the 

 top. The separation of the curd from the serum in the manufacture 

 of cheese involves coagulation followed by precipitation. 



The cream, having separated from the other component parts of the 

 milk in about twenty or two-and-twenty hours, in a medium tempera- 

 ture, is carefully skimmed off, and poured into a vessel, until enough is 

 obtained for churning ; or the milk alone is let off by taking out a plug 

 in the bottom of the pan. When the cream has been thus collected, 

 it should be placed in a deep, covered vessel, for the action of the air 

 on the surface dries it. It should also be stirred with a stick or spoon, 

 every time a fresh quantity is added. The object of this is to ensure 

 uniformity as to ripening. The time of keeping it depends on the 

 weather. If the cream from each milking has been kept separate, it 

 may remain from two to four days, in warm weather, without being 

 injured ; but if sweet cream is mixed with that which is sour, the two 

 ferment and soon become clotted if the churning is delayed beyond 

 three days. This may be in some degree prevented by the stirring ; 

 but it is generally considered best to keep separate the cream from 

 each milking, and thus allow each to become ripe of itself. Cream 

 should be churned before it becomes sour, or the delicate flavour of 

 the butter will be injured. When it is on the point of turning a little 

 sour, it is considered "ripe," and then is the time to churn it. The 

 " ripeness " can be tested with litmus paper. Butter from ripened 

 cream has a flavour more matured than that from sweet cream, and the 

 ripened cream churns all the easier. 



x 



