In order to appreciate the viewpoint which is established by recent researches on foods and nutrition it is essential to fully understand the great advantages which we now enjoy in being able to think of foods in an entirely different way from what was possible a few years ago. Ten years ago the crude comparison of alimentation with the stoking of an engine seemed satisfactory to most teachers. This is no longer a suitable figure for conveying to the student of nutrition the conception which it is desired he should gain. Little more than a decade ago it was supposed that a chemical analysis of a food for protein, carbohydrate, fat and mineral salts, was sufficient to reveal its biological properties. Certain standards of composition had become established on the authority of experts, and it was believed that if any food fulfilled the requirements of these standards with respect to total digestible nutrients, available energy and protein content, it could be counted upon to meet the nutritive requirements of the body. In Chapter II (A Biological Method For The Analysis Of A Foodstuff. 17. Mccollum'S Experiments Not Verified By Osborne And Mendel) it has been described how after the year 1900 many attempts to nourish young animals on food mixtures made up of purified substances, and including the approved amounts of all the components of foods which the chemist was called upon to estimate, proved utter failures, and demonstrated the inadequacy of a time-honored theory.