No technical description of a cheese-handling process can replace experience. Descriptions of appearances and textures of curd in terms definite enough to be understood by beginners have been found to be impossible. It is possible, however, to lay down principles and essentials of practice which are common to the industry and form the foundation for intelligent work. Cheese-making will be a science only as we depart from the mere repetition of a routine or rule-of-thumb practice and understand the underlying principles. +4. Problems in cheese-making.+--Any understanding of these problems calls for a working knowledge of the very complex series of factors involved. These include the chemical composition of the milk, the nature of rennet and character of its action under the conditions met in cheese-making, the nature of the micro-organisms in milk, and the methods of controlling them, their relation to acidity and to the ripening of the cheese. To these scientific demands must be added acquaintance with the technique of the whole milk industry, from its production and handling on the farm through the multiplicity of details of factory installation and organization, to those intangible factors concerned with the texture, body, odor and taste of the varied products made from it. Some of these factors can be adequately described; others have thus far been handed on from worker to worker but have baffled every effort at standardization or definition. +5. History.+--The recorded history of the common varieties of cheese is only fragmentary. Practices at one time merely local in origin followed the lines of emigration. Records of processes of manufacture were not kept. The continuance of a particular practice depended on the skill and memory of the emigrant, who called his cheese after the place of origin. Other names of the same kind were applied by the makers for selling purposes. The widely known names were thus almost all originally geographical. Some of them, such as Gorgonzola, are used for cheeses not now made at the places whose names they bear. Naturally, this method of development has produced national groups of cheeses which have many common characteristics but differ in detail. The English cheeses form a typical group of this kind. Emigration to America carried English practices across the Atlantic. The story of cheese-making in America has been so closely linked with the development of the American Cheddar process that the historical aspects of the industry in this country are considered under that head in Chapter VIII. CHAPTER II _THE MILK IN ITS RELATION TO CHEESE_ The opaque whitish liquid, secreted by the mammary glands of female mammals for the nourishment of their young, is known as milk. The milk of the cow is the kind commonly used for cheese-making in America.