116 VARIOUS FOOD-PLANTS 



indicated. When we were considering the amount of any substance 

 in a given food, we were able to express tlie facts with perfect dcf- 

 initeness because we were deaUng with what could be n^easured by 

 weight and vokunc, and because we had the units (gram and cubic 

 centimeter) by which the measurements could be expressed. Al- 

 though 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. ^ 



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 energ}', 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 kilo- 

 grammeter, we thus have in the expression 1 Calory— 4^4 kilogram- 

 meters, what is known as the "mechanical equivalent of heat." 



43. Energy of vegetable foods. Experiments show that if com- 

 pletely burned, 



1 gram of fat yields 9.3 Calories 



" " "carbohydrate " 4.1 

 " " "proteid " 4.1 



These figures also indicate approximately the amount of energy 

 which would be obtained from equal quantities of the same sub- 

 stances consumed in the human body. To estimate, therefore, 

 the amount of energy obtainable from 100 grams of any food of 

 which we know the chemical composition, we have only to multiply 

 the percentage of each nutrient by the number of Calorics yielded 

 by a single gram, and add the products thus obtained. This has 

 been done for the vegetable foods of which the composition is given 

 in the chemical chart (Fig. 120); and the number of Calories is 

 indicated by heavy lines having lengths proportionate to the amount 

 of energy yielded by the foods they represent. Foods which yield 

 much energy are commonly described as being "hearty": the lines 

 in the chart may be said therefore to indicate the relative "hearti- 

 ness" or fuel- value of common vegetable foods. 



But it may be asked, Does a fat and a carbohydrate serve us in 

 exactly the same way? Physiologists tell us that either may replace 

 the other in our food, provided the amounts eaten represent an 

 equivalent number of Calories; but there is this difference that, 

 whereas carbohydrates (which, so far as they are digestible, enter 

 the blood as sugar) are immediately after digestion available as a 

 source of heat and muscular energy, fats require to undergo some 

 ]")reliminarv transformation in the body, before they can be used, 

 and are therefore less serviceable for immediate needs. Fat, how- 



1 Cal'o-ry < L. caZor, heat. 



