330 HUMAN PHYSIOLOGY 



of protein to the body is a trifle over 4 Calories. It is 

 nearly the same as that of the carbohydrates. 



Calorimetry. A calorimeter is an apparatus for 

 measuring the evolution of heat. It may be adapted 

 to show how much heat is given off by samples of food 

 or other substances when burned in oxygen. It may 

 be of a form to hold a living animal or even a man and 

 to give us data in regard to the evolution of heat that 

 accompanies the metabolic process. A respiration 

 chamber such as has been described may be a calori- 

 meter also. When it has this feature we can obtain 

 simultaneously the material and the dynamic output 

 of the inmate. 



In some of the early and crude attempts to find out 

 how much heat issues from the body of an animal the 

 estimates were based upon observing how much ice 

 could be melted at the expense of the metabolism. This 

 involved chilling the animal and has been abandoned 

 in* favor of better methods. Sometimes the heat is 

 reckoned by recording the rise of temperature in a 

 volume of air which is exposed to the animal's influence. 

 In other cases the heat is absorbed in a known mass of 

 water and the calculation based on the extent to which 

 the temperature is raised. This is the principle followed 

 in the great calorimeters applicable to the human subject. 



It is important to understand that not all the energy 

 produced in the course of the metabolism results in heat 

 that is directly measurable. The most considerable 

 exception is found in the disappearance or making 

 " latent" of a large quantity of heat through the evapo- 

 ration of water. A man in a respiration chamber may 

 easily evaporate a liter (1000 grams) of water in twenty- 

 four hours. This change in so large a mass of water 

 from the liquid to the gaseous state entails the ap- 

 parent annihilation of heat to the amount of more 

 than 500 Calories. (It is not really annihilation, how- 

 ever; the heat becomes tangible again when the vapor 

 is condensed.) 



