Cod-liver oil is next to butter the most digestible animal fat known. The best cod-liver oil is frozen at a low temperature, by which means the stearin is frozen out, and nearly pure olein is left. Traces of iodine have been found in it, and more commonly a small amount of bile, which probably increases its digestibility. The temperature at which a fat becomes hard is a fair guide to its digestibility. Thus we know that beef, and still more, mutton fat, would become solid, under conditions in which bacon dripping is still soft. Where digestion is weak, there may be an instinctive loathing of fat meat; for such persons, especially for children, some other fat should always be substituted. Thus the addition of butter to the potatoes makes up the deficiency. =Butter= forms 3½ to 4½ per cent. of cow’s milk. It is separated from milk by churning, the oil particles being deprived by this means of their albuminous coats. The more completely the butter-milk is separated the longer the butter keeps. It can be kept longer if salt is added, or in hot weather by keeping it under frequently-changed water. Rancidity indicates the decomposition of traces of the fat of butter into its fatty acid and glycerine. Cream contains about 30 per cent. of butter fat, Cheshire cheese 25 per cent., and skim milk cheese 7 per cent. Butter milk differs from skim milk in the presence of lactic acid. It is more digestible than skim milk, the casein being in a more flocculent condition. The odour and flavour of butter are not due to olein and palmitin, the two chief constituents, but to a smaller quantity of butyrin, caproin, and caprylin fats of a much lower series. Ordinary butter contains a considerable proportion of water, and the presence of about 8 per cent. renders it more palatable; if it is over 15 per cent., the butter is considered adulterated. An excessive amount of salt is sometimes present. The most frequent adulteration is the substitution of foreign fats for butter fat, _e.g._ lard, palm oil, rape seed oil, or cocoa-nut oil. Margarine is most frequently used for this purpose.