That man's general health and period of longevity have decreased, while all other branches of science have so vastly increased, is evidence sufficient to justify the assertion that he has not employed scientific methods to the art of living, or at least to those fundamental principles, such as nutrition, motion, and oxidation, which really govern his health and his life. The difference between youth and age, between virility and senility, is in reality a chemical difference only. The difference between the flexible cartilage of youth, and the stiff cartilage of age is one of chemistry. If, by the process of metabolism, the muscles, bones, tissues, and brain-cells can be made to multiply and to reproduce themselves at eighteen, it seems only logical that science should give us the secret by which this same thing could be done at eighty, and if at eighty, why not at a hundred and eighty? It is by no means extravagant to say that if science can teach us the actual demands of the body under the varied conditions of age, climate, and activity, and the means of supplying these demands with only such food elements as are needed, life can be prolonged to what seems to be our natural period of years. Consider the human body as a machine that possesses the power of converting fuel or food into energy, using or expending that energy at will, reproducing itself piece by piece from the same fuel, and casting out the debris and ashes--if all this is done by the body automatically, and its power to act or to do these things depends so completely upon the fuel or the material with which the body has to work, then the question of the _kind_ of fuel, the quantity, how to select it, how to combine it, how to proportion it, becomes at once the most important problem within the scope of human learning. THE PURPOSE OF THIS WORK When we compare man's longevity with other forms of life, and consider that he breathes the same air, drinks the same water, lives under the same sunshine, and that he differs from them chiefly in his habits of eating, the conviction is forced upon us that in his food is found the secret, or the causes of most of his physical ills and his shortened life. All elements composing the human body are well known. Its daily needs are matters of common knowledge. Science has separated the human body into all its various chemical elements or parts, and weighed and named them; it has also analyzed and separated his food or fuel into its various chemical elements or parts, and named these. It would seem, therefore, a most logical step to unite these two branches of science, and to give to the world the dual science of Physio-food Chemistry, or, what I have named Applied Food Chemistry.