622 



PHYSIOLOGY 



It we take into account the great difficulties of such an experiment, we 

 cannot but be impressed with the closeness of agreement between the total 

 output of energy reckoned as heat and measured by the warming of a given 

 volume of water and the total income of energy as estimated from the chemi- 

 cal reactions involved in the metabolic changes which had taken place during 

 four days of the experiment. The important result which comes out in 

 such experiments is that the food-stuffs produce the same amount of energy 



Water 



Water 



Oxygen enters. 



FIG. 319. Diagram to show the principle of the At water -Benedict calorimeter. 

 (After HALLIBURTON.) 



when oxidised in the body as when burnt to the same end-products outside 

 the body, so that it becomes easy in any given research to sum accurately the 

 energy income of the body. 



The Atwater calorimeter has been improved to such an extent by 

 Benedict and his fellow workers that it has practically replaced all other forms 

 for physiological purposes. It consists of a room or chamber with double 

 non-conducting walls. All round the inner wall of the room are fitted coils 

 of pipes through which a stream of water flows. The pipes are fitted with 

 discs so as to take up rapidly heat produced in the room. The current of 

 water is accurately adjusted so as to maintain the temperature of the inner 

 wall constant. As the inner wall and outer wall are kept at the same tempera- 

 ture, no heat is lost to the exterior, the whole of the heat produced by the 

 animal or individual in the chamber being communicated to the water 

 passing through the chamber. The temperatures of the entering and leaving 

 water are taken by accurate thermometers reading to a hundredth or a 

 thousandth of a degree Centigrade. Knowing the amount of water that has 

 passed through in a given time and the difference in temperature during the 

 same time, it is easy to calculate the amount of heat given off by the animal 

 under investigation. It is generally convenient to maintain a constant 

 difference of temperature between the entering and leaving water by appro- 

 priate adjustment of the amount of water passing through the apparatus. 

 The equality of temperature between the inner and outer casing is recorded 



