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greatly influenced by season, nature of food, state of the animals health, and other considerations. While the pro portion of cream to milk in the case of most breeds ranges from one-twentieth to one-tenth, in the case of the celebrated Aldeniey cattle it amounts to as much as from three to four-tenths. Dr Parkes (Practical Hygiene) gives the following as the average composition of unskimmed

milk having a sp. gr. of r030 :

Casein 4 Fat 3 7 Lactin (Sugar of Milk) 5 Salts..; .. 6 Total Solids 13 3 Water 807 On a low average each pint of milk ought to yield a full half-ounce of butter. The fat or butter is disseminated through freshly-drawn milk in minute, clear globules, each of which is enclosed in a thin membranous sac or bag ; and being specifically lighter than the mass of the fluid, the globules gradually rise to the surface, bringing mixed with them a proportion of milky matter, and form cream. Usually the cream is skimmed off the surface of the milk for making butter, but by some the churning is performed on the milk itself without waiting for the separation of the cream. The operation of churning causes the rupture of the oil sacs, and by the coalescence of the fat so liberated butter is formed. Details regarding churning and the preparation of butter generally will be found under Dairy.

Fresh or uusalted butter of good quality should present a rich straw-yellow colour. At ordinary temperatures it has a firm uniform consistency, while it is soft enough to cut and spread easily under the knife without breaking or crumbling. It should possess a faint sweet odour, and a bland, soft, delicate flavour, melting in the mouth without any indication of grittiness. Pure butter is a complex chemical compound, consisting in large part of fats or glycerides of the non-volatile acids, palmitic acid, and butyroleic acid, with occasionally stearic acid. With these there occur small proportions of glycerides of the volatile acids, butyric, capronic, caprylic, and caprinic acid, to which the butter owes its distinguishing flavour and char acteristics, as it has the non-volatile acids in common with other fats, though in different proportions. Butter when unadulterated and prepared with ordinary care should contain at least 85 per cent, of pure fat, the remainder consisting of casein, water, and salt. The casein is derived from milk, which is never perfectly washed out, but in butter of good quality this ought not to amount to more than from 3 to 5 per cent. Water may be present to the extent of from 5 to 10 per cent, without the butter being subject to a charge of adulteration ; and a small proportion of salt is commonly worked into the butter in its prepara tion, but in what is sold as fresh or sweet this should only bo from i to 2 per cent, of the whole weight.

When butter is exposed to the air for some time, especially in warm weather, or in hot, confined situations, it quickly becomes rancid, acquiring thereby a distinct dis agreeable odour and a biting taste, owing to the develop ment of a volatile fatty acid under the influence of a species of fermentation, which is doubtless caused by the nitrogenous substance, casein, it contains. The more completely, there fore, all milky and curdy matter is washed out of butter the less will be the tendency to sot up and develop fermenta tion. The preservation or curing of butter depends for its efficacy on the employment of some agency by which fermentative action may be prevented ; but there are also several ways by which its development may be retarded and the material kept sweet for a considerable period. Rancidity may be corrected to some extent by melting the affected butter and pouring it into ice-cold water. As a means also of retarding rancidity, butter is in some parts of France and the East melted up and heated till the water it may contain is evaporated, when the casein which rises as a scum to the surface is carefully skimmed off ; but butter cannot be so melted without injuriously affecting its delicate flavour. By keeping fresh butter in a very cool place covered with pure water renewed daily, it will remain sweet for a considerable time. A still better method, recommended by M. Payen (Substances Alimentaires) is to use water acidulated with either tartaric acid or vinegar. It is also said that sugar in the form of a syrup poured over the butter is an excellent medium for retarding rancid fermentation. Butter, however, which is to be kept for a considerable length of time is " cured," or preserved by incorporating with it some substance or substances which act upon the nitrogenous material it contains, and thus prevent fermentation ; and for this purpose common salt is the agent chiefly relied on. The salt used should be pure, dry, and finely powdered. About 5 per cent, of salt is sufficient for the purpose of curing ; and when the quantity exceeds 8 per cent, it ought to be regarded as an adulteration. Butter very lightly salted for keeping only a short time is said to be powdered. A mixture much used for curing butter in Continental dairies is thus prepared: One part each of sugar and nitre are mixed up with two parts of common salt and reduced to a very fine powder. This mixture is thoroughly kneaded into the butter in the proportion of about 1 oz. to every lb. After standing over for a fortnight butter so prepared will be ready for use and have a soft, agreeable taste, which it will retain a long time. In the preservation of all butter, the exclusion of air, as much as possible, is of the utmost consequence. It is, therefore, packed for sale in oaken kegs or glazed earthenware jars, filled quite full, and covered with a clean linen cloth on which salt is sprinkled. When in use the kegs should also be closely covered over, and the surface of the butter kept under brine.

Butter of good quality is a most digestible form of fat, while its flavour is so delicate and little pronounced that it is always acceptable to the palate. It is used most extensively by all classes, not only in the direct form with bread at nearly every meal, but also as entering very largely into the preparation of pastry, puddings, sauces, fancy cakes, and biscuits. Taking into account the daily con sumption of this article, it is evident that the amount used in a year by a population such as that of Great Britain must be very great, an inference borne out by the fact that in 1875 the imports were 1,619,808 cwts., valued at 9,050,025, and, though no means exist of accurately estimating the home produce, it may safely be regarded as equal to the whole imports. The countries whence butter is imported into Great Britain are chiefly Germany, Holland, Denmark, and especially France. A large proportion of the French butter comes from the department of Calvados Isigny being the centre of the best butter-making district. The value of the total produce of France in 1867 was estimated at 250 millions of francs.

Butter is a substance which affords great scope for

adulteration, and its composition makes accurate detection of certain foreign matters a matter of considerable labour and difficulty. Other animal fats, such as lard, beef and mutton dripping, and tallow, with certain vegetable fats, are the chief adulterants. Such adulterations may be suspected by their characteristic smell, and detected by their different melting points, by microscopical examination, and by their ethereal solutions. Messrs Angell and Hehner have pro posed a convenient method of estimating the fusing points of fat by placing a given weight of definite size on the fat, and observing the temperature at which it sinks Into the sub

stance. They find that the sinking-point for genuine butter