When a scientist states that a hen cannot lay egg shells containing calcium without having calcium in her food, that is a fact, and it works out in practice, for calcium is an element, and the hen cannot create elementary substances. When the same scientist, finding that an egg contains protein, says that wheat is a better egg food than corn because it has the largest amount of protein, that is a guess and does not work in practice because protein is not a definite substance, but the name of a group of substances of which the scientist does not know the composition, and which may or may not be of equal use to the hen in the formation of eggs. All substances of which the world is made are composed of elements which cannot be changed. When these elements are combined they form definite substances with definite proportions entirely independent of the original elements. The pure diamond is carbon. Gasoline is carbon and hydrogen. Several hundred other things are also carbon and hydrogen. Sugar is carbon combined with hydrogen and oxygen. These three elements make several thousand different substances, including fats, alcohol and formaldehyde. Hydrocyanic acid is carbon combined with hydrogen and nitrogen, and is the most deadly poison known. The failure of food science is partly because we do not know the composition of many of the substances of food and partly because these substances are changed in the animal body in a manner which we do not understand and cannot control. Conventional Food Chemistry The conventional analysis of feeding stuff divides the food substances in water, carbohydrates, fat, protein and ash. The amount of water in the body is all-important, but, with the exception of eggs during incubation, I confess I prefer to rely upon the chicken's judgment as to the amount required. The carbohydrate group contains starch, sugar, cellulose and a number of other things. Carbohydrates constitute two-thirds to three-fourths of all common rations and nine-tenths of that amount is starch. The proposition of how much carbohydrates the hen eats is chiefly determined by the quantity of grain she consumes. Of fats there are many kinds of which the composition is definitely known. The amount of fats the hen eats is unimportant because she makes starch into fat. The protein or nitrogen containing substances of the diet is the group of food substances over which most of the theories are expounded. The hen can make egg fat from corn starch or cabbage leaves because they contain the same elements. She cannot make egg white from starch or fat because the element of nitrogen which is in the egg white is lacking in the starch and fats.