Butter is the fatty constituent of milk, wherein it exists suspended in the form of minute globules. When the liquid is left at rest, these, together with other substances, rise to the surface and form a layer of cream. The butter is formed on agitating the cream, when an aggregation of these globules ensues. Commercial butter also contains certain proportions of water and curd, the latter being the cause of the butter becoming rancid. Butter is composed of fatty acids in combination with glycerine, the most important of these being oleic, palmitic and butyric acids, while it is often adulterated with an excess of water and salt or a mixture of ordinary animal and vegetable fats.