116 VARIOUS FOOD-PLANTS 



indicated. When we were considering the amount of anj' substance 

 in a given food, we were able to e.xpress the facts with perfect def- 

 iuiteness 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. 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 olataincd 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 reciuired to lift 1 kilogram, 1 meter, being called a kilo- 

 grammeter, we thus have in the expression 1 Calory=z^24- kilogram- 

 meters, what is known as the "mechanical ec|uivalent of heat." 



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

 pletely burned. 



These figures also indicate approximately the amount of energy 

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

 stances consumed in the human bod}'. To estimate, therefore, 

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

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

 the percentage of each nutrient by the number of Calories 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 hea^'y lines ha\'ing 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 -^-egetable foods. 



But it maj' be asked, Does a fat and a carljohydrate serve us in 

 exactly the same way? Physiologists tell us that either man 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 

 preliminary transformation in the liody, before they can be used, 

 and are therefore less scr\'iceal)le for inunetliate needs. Fat, how- 



' Cal'D-rv < 1.. rahir, heat. 



