ANIMAL CALORIMETRY 27 



(ii) Certain substances are excreted in conibinatioii with protein 

 disintegration products, e.g. hip{)uric acid (protective syntheses). 



(c) Certain substances or their disintegration products seem 

 to be necessary constituents of faeces, e.g. fats and soaps (see 

 Chap. XXVIII.). 



The energy- value of all excreta must therefore be deducted from 

 the energy intake before an energy balance can be struck. 



C, Measurement of the E.V. of foods by animal ealorimetry — (a) direct, 

 (b) indirect. 



It is obvious as a direct deduction from the first law of energetics 

 that if this law holds in living as well as in non-living matter- 

 energy transformations, the same amount of energy should be 

 evolved from the utilisation of food inside as well as outside the 

 body, provided always that the physical state and chemical 

 end-products are the same in each case. If an animal could be 

 put inside a calorimeter and given a definite amount of food, 

 the heat evolved should, providing our hypothesis is true, be 

 exactly the same as would be evolved in direct food ealorimetry. 

 Each gram of carbohydrate should produce 4-1 Cals., and so on. 

 This can be put to the test in either of two ways. The first is 

 known as direct (animal) ealorimetry, and consists in accurately 

 measuring the heat evolved by the animal under investigation. 

 The second or indirect method is based on a knowledge of the 

 amount of heat evolved per litre of the respiratory gases and per 

 gram of urinary nitrogen. 



(a) The direct method was first employed by Crawford (1779). 

 His calorimeter, in principle, consisted of a double-w^alled box 

 with a known amoimt of water between the walls. The animal 

 was placed in the inner box for a definite time and the increase 

 in temperature of the water noted at the end of the experiment. 

 The method is, of course, primitive, and the veriest tyro in physics 

 could suggest quite a host of sources of error, but on this crude 

 instrument are based those finer implements of research which, 

 in the hands of Benedict and his colleagues, have contributed 

 so much to the knowledge of nutrition. Crawford foimd that 

 for every 100 ozs. of oxygen used during the combustion of carbon 

 in his calorimeter, the temperature of the water was raised 1-93° F. 

 A live guinea-pig consuming the same amount of oxygen produced 

 an increase of 1-73° F. This seemed sufficient evidence for him to 

 conclude that, in each case, the heat produced was due to the 

 conversion of pure air into fixed air, or, as we should now say, to 

 the combination of C and Og. 



A year later, Lavoisier and Laplace published the result of 



