116 VARIOUS FOOD-PLANTS 
indicated. When we were considering the amount of any substance 
in a given food, we were able to express the facts with perfect def- 
initeness 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. AlI- 
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 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 kilo- 
grammeter, we thus have in the expression 1 Calory=424 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 
c «&~ “ carbohydrate oa” ae i 
‘c “ ‘c proteid cc 4.1 “c 
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 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 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 
preliminary transformation in the body, before they can be used, 
and are therefore less serviceable for immediate needs. Fat, how- 
1 Cal’o-ry < L. calor, heat. 
