LECTURE XX. 



Of milk and its products.— Properties and composition of the milk of different animals.— 

 Circumstances which aflFect the quality and quantity of milk— species, size, variety, age, 

 health, and constitution of the animal, time of milking, kind of food, &c.— Mode of sepa- 

 rating and estimating the several constituents of milk. — Sugar of milk, and acid of milk 

 (Lactic acid), their composition and properties. — Souring of milk, cause of. — Cream — 

 composition and variable proportions of— mode of estimating iis quantity — the gaiactome- 

 ter. — Churning of milk and cream. — Composition of butter. — ButKr-milk. — The solid and 

 liquid fats contained in butter — margarin and butter-oil — their separation and properties- — 

 Rancidity and preservation of butter.— Composition and propertits of the curd ^casein). — 

 Curdling of milk, natural and artificial — by acids and by animal membranes. — Making and 

 action of rennet — how explained. — Manufacture of cheese. — Varieties of cheese. — Aver- 

 age produce of butter and cheese.— Colouring of butter and cheese.— The whey.— Saline 

 matter in the whey. — Nature of the saline constituents of milk. — Fermentation of milk. — 

 Intoxicating liquor from milk. — Milk vinegar. — Purposes served by milk in tlie economy 

 of nature. 



Or the indirect products of agriculture, milk, and the butter and 

 cheese manufactured from it, are among the most important. In our 

 large towns these substances may almost be considered as necessaries of 

 life, and many extensive agricultural districts are entirely devoted to the 

 production of them. The branch of dairy husbandry also presents many 

 curious and interesting questions to the scientific enquirer, and upon 

 these questions modem chemistry has thrown much light. To the con- 

 sideration of this subject, therefore, it is my intention to devote the pre- 

 sent lecture. 



§ 1. Of the ]noperties and composition of milk. 

 1°. Properties of milk. — The milk of most animals is a white opaque 

 liquid, having a slight but peculiar odour — which becomes more distinct 

 when the milk is warmed — and an agreeable sweetish taste. It is 

 heavier than water — usually in the proportion of about 103 to 100.* 

 When newly taken from the animal, cow's milk is almost always 

 slightly alcaline. It speedily loses this character, however, when ex- 

 posed to the air, and hence even new milk often exhibits a slight degree 

 of acidity. f When left at rest for a number of hours, it separates into 

 two portions, throwing up the lighter part to the surface in the form of 

 cream. If the whole milk, or the cream alone, be agitated in a proper 

 vessel (a churn), the temperature of the liquid undergoes a slight increase, 

 it becomes distinctly sour, and the fatty matter separates in the form of 

 butter. If a little acid, such as vinegar or diluted muriatic acid, be add- 

 ed to milk warmed to about 100° F., it immediately coagulates and se- 

 parates into a solid and a liquid part — the curd and the whey. The 

 same effect is produced by the addition of rennet or of sour milk — and 

 it takes place naturally when milk is left to itself until it becomes sour. 

 At a very low temperature, or when kept in a cool place, milk remains 

 sweet for a considerable time. At the temperature of 60° F. it soon 



* Or it has a specific gravity of 1020 in woman's milk, to 1041 in sheep's milk ; water being 

 1000. 



t It is said that if the animal remain long unmilked, the milk will begin to sour in ttie 

 udder, and that hence it is sometimes slightly acicTwhen fresh drawn from tbe cow. 

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