Experience shows that a definite amount of heat will yield a definite amount of work, so that the amount of heat produced by a given amount of food, when combined with oxygen, is taken as a measure of its energy. This is ordinarily expressed in calories, a calorie being the amount of heat required to raise the temperature of one thousand grams of water one degree on the centigrade thermometer scale. The use of these terms need not concern the student. Instead of using the calorie I will use a unit which is equal to one hundred calories. I have selected a unit of this size because it gives about the ordinary service of food at meals which is easily measured and remembered. NITROGEN Nitrogen is the chemical element that is most concerned with the function of life. All animal tissue contains nitrogen, which forms about one-sixth part, by weight, of all the nitrogenous or protein substances. [Sidenote: Proportion of Nitrogen in lean meat] If we were to take a hundred pounds of lean meat, or muscle, and evaporate from it all the water, we would have about eighteen pounds of dry material left. If we should analyze this dry substance, we would find that about one-sixth, or three pounds, would be the element nitrogen. Thus we say that muscle contains eighteen per cent of protein, or three per cent of nitrogen. In ordinary practise the protein is mixed with fats and salts, and cannot be measured by simply drying out the water, so the chemist finds the amount of nitrogen present and multiplies by 6.25, which gives about the correct per cent of protein. This method is not exact because the per cent of nitrogen in various proteids is not always the same, but it will give an intelligent average. I will discard the use of the term protein, and refer to the amount of nitrogen directly. All compounds of the element nitrogen are not available as food. For example: The nitrogen of the air, of ammonia gas, or gunpowder cannot be utilized in the animal body. The nitrogen in foods only refers to available nitrogen. Compounds containing other forms of nitrogen are not foods, but are frequently poisons. SYSTEMS OF FOOD MEASUREMENTS COMPARED THE "OLD" SYSTEM Under the old system of food measurement, feeding the human body cannot be made a practical science for the masses, therefore a new system becomes necessary. That we may more fully appreciate the value of a new system, let us consider the methods hitherto available. Suppose a man is using two quarts of milk a day, and wishes to determine the amount of available nitrogen or tissue-building material and energy it contains. Under the old system he must get a book on food analysis, or send to Washington for a Government bulletin. If he does not understand the meaning of the terms and figures used, the tables would be useless to him until he goes to a chemist to have them explained. He is now ready to work out the nutritive value of his milk, and proceeds as follows: