The proportion of fat present in genuine butter ranges from 82 to 90 per cent.; it should never be below 80 per cent. The average amount of caseine is 2·5 per cent.; greater proportions, frequently occurring in unadulterated butter, render it more liable to become rancid. The ash should consist of sodium chloride, with some calcium phosphate; the amount of salt is quite variable, but it usually ranges from 2 to 7 per cent. The proportion of ingredients, not fat, in butter may be conveniently determined by melting 10 grammes of the sample in a graduated tube, provided with a scale at its lower end, which is narrowed, adding 30 c.c. of petroleum naphtha, and shaking the mixture. After standing a few hours, the non-fatty matters collect in the lower portion of the tube, and their volume is read off. Genuine butter is said to yield from 12 to 14 per cent. (assuming each c.c. to equal one gramme), while adulterated specimens may show 20 per cent. of matters not fat. _Examination of the Butter-fat._ The most common and important sophistication of butter consists in the addition of foreign fats, embracing both animal fats (oleomargarine and lard) and vegetable oils (cotton-seed, olive, rape-seed, cocoa-nut, almond, palm, etc.). Of these, oleomargarine is doubtless the most often employed. Oleomargarine is the more fusible portion of beef fat, and is prepared by straining the melted fat, allowing the oil thus obtained to stand for some time at a temperature of about 24°, when most of the stearine and palmitine will separate out, and cooling the remaining oil until it solidifies. This is next churned with milk, a little colouring matter (annato) being added, and the product is then chilled by mixing it with ice; salt is now added, and the mass is finally worked up into lumps. It is stated that fifteen establishments in the United States are engaged in the manufacture of oleomargarine, the annual production in the State of New York alone being about 20,000,000 pounds. The rapid increase in the manufacture of oleomargarine is shown by the following statistics:--In 1880 this country exported 39,236,655 pounds of butter and 20,000,000 pounds of oleomargarine, while in 1885 the exportation of butter declined to 21,638,128 pounds, and the exportation of oleomargarine reached nearly 38,000,000 pounds. The present production is said to approximate 50,000,000 pounds per annum. The most characteristic difference in the composition of genuine butter and oleomargarine consists in the greater proportion of soluble fats contained in the former. This is illustrated by the following comparative analysis of the two products (Mége Mouriès):--