28 CALORIMETRY 



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

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

 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) calorimetry, 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-walled box 

 with a known amount 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 con- 

 tributed so much to the knowledge of nutrition. Crawford found 

 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 suffi- 

 cient evidence for him to conclude that, in each case, the heat 

 produced was due to the conversion of pure air into fixed, or, as 

 we should now say, to the combination of C and O 2 . 



A year later, Lavoisier and Laplace published the result of 

 experiments which confirmed Crawford's results, and made firm 

 the principle of indirect calorimetry. They determined the 

 amount of ice melted by the combustion of a weighed amount of 

 carbon (a candle) and the volume of the CO 2 evolved. A similar 

 experiment was then tried with a guinea pig. They found that 

 for equal volumes of CO 2 formed, the candle yielded 25-4 cals. 

 as against the guinea pig's 31-8. The experiment is bristling 

 with errors, many of which the authors realised. For instance, 

 the respiratory and calorimetric determinations were not, as by 

 Crawford, made simultaneously, and obviously thermal con- 

 ditions were not the same. As we shall see later, cold raises the 

 CO 2 output. If allowance is made for this and for other minor 

 errors, the figures for candle and animal come close enough to 

 justify the conclusion that the processes are similar, and that 

 the source of heat in both is the combination of C and O 2 . 



