59 8 ANIMAL HEAT. | CH. XL. 



The figures under the heading Production are obtained by 

 multiplying the weight of food by its physiological heat-value. 

 The figures on the other side of the balance-sheet are obtained as 

 follows : The water in the food is reckoned as weighing 2 '6 kilos. 

 This is supposed to be at the temperature of the air taken as 

 12 C. ; it has to be raised to the temperature of the body, 37C., 

 that is, through 25 C. Hence the weight of water multiplied by 

 25 gives the number of calories expended in heating it. The 

 weight of air is taken as weighing 16 kilos. ; this also has to be 

 raised 25 C., and so to be multiplied by 25 ; it has further 

 to be nmltiplied by the relative heat of air (o - 24). The 630 

 grammes of water evaporated in the lungs must be multiplied 

 by the potential or latent heat of steam at 37 C. (582) ; the 

 portion of heat lost by radiation, conduction, and evaporation 

 from the skin constitutes about four-fifths of the whole, and is 

 obtained by deducting the three previous amounts from the total. 

 This table does not take into account the small quantities of heat 

 lost with urine and faeces. 



It need hardly be remarked that the above is a mere illustra- 

 tive experiment. Changes in the diet, in the atmospheric tem- 

 perature, in the temperature of the food taken, in the activity 

 of the sweat-glands, in the amount of moisture in the atmosphere, 

 and in the amount of work done would considerably alter the 

 above figures. 



Galorimetry. Calorimeters employed in chemical operations 

 are not suitable for experiments on living animals. An animal 

 surrounded by ice or mercury, the melting and expansion of which 

 respectively are measures of the amount of heat evolved, would be 

 under such abnormal conditions that the results would be valueless. 

 Lavoisier, however, used an ice calorimeter in his experiments on 

 animals. 



The apparatus often employed is the water calorimeter. This 

 was first used by Crawford (i 788). Dulong's instrument is shown 

 in fig. 447. The animal is placed in a metal chamber, surrounded 

 by a water-jacket. There are also tubes for the entrance and 

 exit of the inspired and expired gases respectively. The heat 

 given out by the animal warms the water in the jacket, and is 

 measured by the rise of temperature observed in the water, of 

 which the volume is also known. The air which passes out from 

 the chamber goes through a long spiral tube, passing through the 

 water-jacket, and thus the heat is abstracted from it and not lost. 



Air-calorimeters are now, however, generally used. Fig. 448 

 is an outline sketch of the one which has been most used in this 

 country. 



