Some scientists include air in the list, but it has been thought best in this work to speak of it separately as the greatest necessity of life, but not in the sense of a direct nutrient. An average composition of three of the principles is as follows: { Carbon 53 PROTEIN { Hydrogen 7 { Oxygen 24 { Nitrogen 16 { Carbon 76.5 FATS { Hydrogen 12 { Oxygen 11.5 { Nitrogen -- { Carbon 44 CARBOHYDRATES { Hydrogen 6 { Oxygen 50 { Nitrogen -- It will be seen from the above that the protein compounds contain nitrogen; the fats and carbohydrates do not. WATER We will now consider the first of the food principles--water. Water is one of the necessities of life. A person could live without air but a few minutes, without water but a few days. It constitutes by weight three fifths of the human body, and enters largely into all organic matter. Water is an aid to the performance of many of the functions of the body, holding in solution the various nutritious principles, and also acting as a carrier of waste. It usually contains foreign matter, but the nearer it is to being pure the more valuable it becomes as an agent in the body. Ordinary hydrant, well, or spring water may be made pure by filtering and then sterilizing it. =Exp.= Put a little water into a test-tube, and heat it over the flame of an alcohol-lamp. In a short time tiny bubbles will appear on the sides of the glass. These are not steam, as may be proved by testing the temperature of the water; they are bubbles of atmospheric gases which have been condensed in the water from the air; they have been proved to be nitrogen, oxygen, and carbonic acid, but as they do not exist in the water in the same proportions as in the air, they are not called _air_, but _atmospheric gases_. Continue the heating, and the bubbles will continue to form. After a while, very large bubbles will appear at the bottom of the tube; they increase rapidly and rise toward the top; some break before reaching it, but as the heat becomes more intense others succeed in getting to the surface,--there they break and disappear. If the water now be tested with a thermometer, it will be found to have reached 212° Fahrenheit or 100° Centigrade, provided the experiment be tried at or near the level of the sea.