It has been found,mat there is an exact quantitative relation betwen heat and work, expressed by the term mechanical equivalent of heat. Experiments have shown that about 778 foot-pounds of work are consumod in heating one pound of water one degree Fahrenheit, or 1400 foot-pounds in heating the same amount of water one degree Centigrade. In other words, the same amount of energy would be expended in heating a pound (about one pint) of water one degree Fahrenheit, as in raising a weight of 778 pounds one foot, or a weight of one pound 778 feet. By the same calculations a calorie is equivalent to 3,087 foot-pounds. The calorie then is used as a convenient measure not only of quantity of heat, but of mechanical -energy, or pozver to work.
One gram of proteid has been found to yield 4.1 calories; a gram of carbohydrate yields the same amount, while a gram of fat yields 9.3 calories. Or more than twice as much heat can be obtained from a given amount of fat as from the same amount of either proteid or carbohydrate.
The "Bomb" of a Bomb Calorimeter.
The number of calories any particular food will yield theoretically is determined by the • use of the bomb calorimeter. A portion of .food of a given weight is enclosed in an iron shell or "bomb," which is then immersed in a given amount of water and the temperature of the water taken. By means of an electric spark the contents of the bomb are ignited and burned, and the temperature of the water is again taken at the end of the combustion. For instance, if the burning of one gram of meat raised the temperature of one kilogram (about two pounds) of water seven degrees Centigrade, that amount of meat would be said to yield seven calories.
The chart given shows the number of calories yielded by several different foods.
There is one factor that is often not sufficiently considered in determining the amount of energy obtainable from food. A food may yield excellent results in the calorimeter and yet be of little service in the body because of its lack of digestibility. It is by no means the food we eat but the food we assimilate that nourishes us. The portion of food that is really absorbed by the body differs greatly under different conditions and with different food materials. Many careful experiments have been made of late, and more will be made to determine the amount assimilated in different cases. This element of digestibility is frequently not taken into account, and the value of a food is estimated wholly from its chemical composition. Some reasons for this are the great difficulty in determining the digestibility of a food, the fact that this digestibility may vary from time to time according to the condition of the body, and the fact that the personal equation enters largely into the matter.