56 PHYSIOLOGY CHAP. 



with approximate and average figures, since it is almost impossible 

 to specify the quality and quantity of the different proteins, fats, 

 and carbohydrates of which our articles of food are composed. 

 For the purpose of such estimations Eubner has calculated from 

 the preceding exact determinations the average physiological 

 values for the heat of combustion of the three principal groups of 

 organic food substances; they may be used without any great 

 risk of error in all cases in which we know the total amount of 

 protein, fat, and carbohydrate taken in the food. According to 

 Kubner, 1 grm. of protein yields 41 cal., 1 grin, of fat 9 '3 cal., and 

 1 grm. of carbohydrate 4*1 cal. On the principle of the conserva- 

 tion of energy, these theoretical values should correspond to the 

 heat actually developed by the combustion of the said substances 

 within the living orgauism. The direct proof of the correctness 

 of this deduction Rubner obtained by ascertaining in dogs the 

 total exchange of material, the estimated heat value of the three 

 groups of food substances given to them, and by direct determina- 

 tions the amount of heat developed during the same period. A 

 comparison of the production of heat calculated from the exchange 

 of material with that found directly with the calorimeter showed 

 in eight series of researches spread over a period of forty-six days 

 a difference of only 0'30 cal. per cent. 



Atwater and Benedict repeated such researches on men kept 

 upon a normal mixed diet. They not only analysed the intake 

 and output, but also determined the heat of combustion for both, 

 and at the same time measured the loss of heat by means of a 

 specially constructed calorimetric chamber. 



Many forms of physiological calorimeters have been suggested for the 

 measurement of the heat produced by animals and man under normal and 

 pathological conditions. Tne most important ones will be considered. 



The general principle common to all these calorimeters is that the source 

 of heat the animal must be so placed that it produces variations of tempera- 

 ture in a suitable medium, the calorimetric medium. The exact measurement 

 of such variations, or a measurable effect of the said variations, affords the 

 data for the calculation of the heat produced by the animal. 



The different calorimetric methods are classified according to the nature 

 of the calorimetric medium A solid, B liquid, and C gaseous calorimetric 

 medium. 



A. Of the apparatus of the first type we may mention, because of its 

 historical importance, the one for small animals constructed by Lavoisier and 

 Laplace, in which ice is used as the calorimetric medium. The heat 

 produced by the animal is used to melt the ice, the heat produced being 

 calculated from the resulting water. Fig. 6 shows the calorimeter, which 

 consists of three concentric cavities, of which the innermost, A, contains the 

 animal ; the middle one, B, filled with pieces of ice at is provided at the 

 bottom with a tube having a tap D, from which the water resulting from the 

 melting of the ice by the heat produced by the animal runs into the vessel G. 

 The third or outer cavity G is also filled with ice, and serves to isolate the 

 second from the external temperature. Naturally, only the water which 

 collects in G, as the result of the melting of the ice by the heat of the 

 animal, is taken into account. 



