The second of the food principles, protein, is a complex and very important constituent of our food. The protein compounds differ from all others as to chemical composition by the presence of nitrogen; they contain carbon, oxygen, hydrogen, and nitrogen, while the fats and carbohydrates are composed principally of carbon, oxygen, and hydrogen, but no nitrogen. The so-called extractives or flavoring properties of meats are nitrogenous, and are consequently classed with the protein compounds.2

The body of an average person contains about eighteen per cent, of protein. The proteins of various kinds furnish nutriment for blood and muscle, hence the term "muscle-formers," which is sometimes given them. They also furnish material for tendons and other nitrogenous tissues. When these are worn out by use, it is protein which repairs the waste.

l As a general thing water does not contain organisms that form spores.

2 Atwater.

Most of the valuable work upon the analysis of food has been done in Germany. From estimates made by chemists of that country it has been decided that the amount of protein in a diet should not fall below four ounces daily. This is to represent an allowance for a man of average weight doing an average amount of work, below which he cannot go without loss in health, in work, or in both. Although protein is the most expensive of all food materials, one should endeavor to use at least four ounces each day. Meat, milk, eggs, cheese, fish of all kinds, but especially dried cod, wheat, beans, and oatmeal are all rich in this substance. The protein compounds are divided into three classes: