838 THE POTENTIAL ENERGY OF FOOD. [BOOK n. 



calculations in question. And indeed when in an animal we on the 

 one hand directly determine the total energy expended during a given 

 time, say twenty-four hours, and on the other hand the potential 

 energy supplied by the food actually oxidized during that time, we 

 find the two sums agree so closely as to be practically identical. 

 On the one hand we can estimate the energy expended as heat by 

 a calorimeter, and this with the smaller amount of energy ex- 

 pended in work done will give us the total energy expended. On 

 the other hand we can, as we have seen, 521, determine from the 

 excreta how much food, or rather how much tissue, how much 

 proteid, and how much fat or glycogen, has been oxidized into 

 urea, carbonic acid and water within the body during the same 

 time. And the energy calculated, by the latter way, to have been 

 expended will be found to be identical in amount with the energy 

 determined by the former way. 



The total combustion of the following substances has given for 

 one gramme of each substance the following results expressed in 

 calories, that is in gramme-degree units of heat. 



Proteids. The purified proteids vary, according to the more 

 recent determinations, from 5918 calories for serum-albumin to 

 5299 for peptone. (This difference in potential energy between 

 the native albumin and the derivative peptone is in itself worth 

 noticing.) The various determinations give as the potential energy 

 of what may be considered as the average proteid, 5711 calories. 

 Proteid in the form of meat, necessarily much more variable, has 

 been determined when as free as possible from fat at from 5343 to 

 5778. 



Fats. The fat of adipose tissue, which though it varies in 

 nature in different animals has practically the same elementary 

 composition, and possesses the same potential energy, may accord- 

 ing to the most recent determinations be put down as 9500. The 

 fat of butter has a lower potential energy, 9231. 



Carbohydrates. Dextrose, 3743. Cane-sugar, 3955. Starch, 

 4183. Glycogen, 4191. Cellulose, 4190. 



From the potential energy of the proteid we must deduct the 

 potential energy of the urea to which it gives rise. We have 

 seen ( 507) that 1 grm. of proteid gives rise to about Jrd grm. 

 urea, more exactly to '3428 grm. Now 1 grm. urea gives by 

 complete combustion 2537 calories; we have therefore to deduct 

 from the 5711 calories, the energy of the proteid, 869'7 calories, 

 that is to say about 15 p.c. of the total, reducing it to 4841 '3. 



But still other deductions have to be made. Part of the food 

 goes to form the faeces, and these being combustible, represent a 

 certain amount of energy present in the food but not used by the 

 body. In this respect we have to consider the whole faeces, both 

 the part which has come immediately from the food, undigested 

 material, and the part supplied by the secretions, derived from the 

 food in an indirect manner ; both these represent energy of which 



