CALORIE VALUES FROM COMBUSTION 25 



r('(|uires some correction. Finally, to obtain tlie correct content 

 of the foodstuff of ('. H and O re(|uires a complicated chemical 

 techni(]ue, and the calorie \alue of an element may vary witii its 

 position in the molecule. For example, Barker states that 



the E.V. of the CO group is 60-7 Calories 

 and „ „ OH „ 12-9 „ 



but the EV of a COOH group may not be obtained by adding 

 60-7 and 12-9. This investigator finds that carbon has lower 

 calorie values when it is arranged in such a position in an organic 

 chain that its bonds approach the tetrahedral position. This 

 method of calculating energy-value is little used in biology. Pure 

 substances are seldom used as food material, except in certain 

 kinds of experiments. 



B. Measurement of E.V. of foods by calorimetric combustion. 



The principle underlying this method is the combustion of a 

 known amount of the material in an apparatus so devised that 

 practically all the heat evolved is absorbed by a known amount of 

 water and by the apparatus itself (which is of known heat capacity). 

 Some form of bomb calorimeter is now universally employed for 

 this purpose. The instrument (Fig. 3) is described on p. 24. 



On p. 5 we mentioned the Law of Hess, which enables us to 

 apply calorimetric combustion values to the food used by an 

 animal. Provided the final products of combustion in the boml) 

 are the same as the final products of metabolism, the energy 

 liberated from equal quantities of the same food material will be 

 the same. In the bomb the process of oxidation is rapid, in the 

 bodv it is slow. In the former case the intermediate steps are 

 known, in the latter they are not, but if the carbon becomes CO2 and 

 the hydrogen HgO in each instance, the same quantity of energy 

 will have been evolved. This law also permits us to calculate the 

 energy set free when the process stops before oxidation is complete. 

 For example, if C were only to combine with one instead of two 

 molecules of oxygen and we found from actual combustion that : 



(1) C + O2 = 94-3 Cals. 



and (2) i(2C0 + O2) = 68-1 Cals. 



Then (1 — 2) = 26-2 Cals. would be the heat liberated in the 

 reaction i(2C + O2) = CO. This principle is of great importance 

 in arriving at a caloric value for proteins (see p. 26). 



Determinations of the energy-value of the proximate principles 

 of food have shown that only minute differences exist betw'een 

 the various members of any class. For example, one gram of 



