42. Measures of energy. As we have to depend for warmth and strength mainly upon fats and carbohydrates, it becomes important to inquire how these compare with each other in fuel value, for as already shown, these substances are to our bodies essentially as coal to a steam-engine. It was stated in the last chapter that fats afford more than twice as much energy as carbohydrates. We must now try to understand more fully what this means and at the same time secure a more exact expression of the relation thus vaguely indicated. When we were considering the amount of any substance in a given food, we were able to express the facts with perfect definiteness because we were dealing with what could be measured by weight and volume, and because we had the units (gram and cubic centimeter) by which the measurements could be expressed. Although neither heat nor mechanical force have weight or volume, they may nevertheless be measured as to their amount by means of suitable units. Such a unit for heat is the amount required to raise the temperature of one kilogram of water one degree of the centigrade thermometer. This amount of heat is termed a Calory.1

From very careful experiments it has been calculated that if by means of a steam-engine, one Calory obtained from fuel could be entirely converted into mechanical energy, this would be sufficient to lift a weight of 424 kilograms, 1 meter, or 1 kilogram, 424 meters. The energy required to lift 1 kilogram, 1 meter, being called a kilogrammeter, we thus have in the expression 1 Calory-424 kilogrammeters, what is known as the "mechanical equivalent of heat."

1 Cal'o-ry - L. calor, heat.