A handbook of invalid cooking

by Mary A. Boland

THE FIVE FOOD PRINCIPLES

For convenience of comparison foods may be divided into five classes: Water, Protein, Fats, Carbohydrates, Mineral Matters.

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.

Steam. The large bubbles are bubbles of steam, or water expanded by heat until its particles are so far apart that it ceases to be a liquid and becomes a gas. True steam is invisible; the moisture which collects on the sides of the tube and is seen coming out at the mouth is partially condensed steam, or watery vapor. Watch a tea-kettle as it boils on a stove; for the space of an inch or two from the end of the spout there seems to be nothing; that is where the true steam is; beyond that, clouds of what is commonly called steam appear; they are watery vapor formed from the true steam by partial condensation which is produced by its contact with the cool air.[7]