44 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE,, ETC. 



electric lamp placed in the cross immediately below the tension equalizer J. 

 Cooling was effected by means of a current of water through the lead pipe 

 E closely wrapped around the air-pipe, water entering at F and leaving 

 at G. This lead pipe is insulated by hair-felt pipe-covering, C. More 

 recently, we have adopted the procedure of passing a continuous current of 

 water, usually at a very slow rate, through the lead pipe E and always 

 heating the air somewhat by means of the lamp, the exact temperature con- 

 trol being obtained by varying the heating effect of the lamp itself. This 

 has been found much more satisfactory than by alternating from the cool- 

 ing system to the heating system. In the case of the air-current, however, 

 it is unnecessary to have the drop-sight feed-valve as used for the wall con- 

 trol, shown in fig. 13. 



THE HEAT OF VAPORIZATION OF WATER. 



During experiments with man not all the heat leaves the body by radia- 

 tion and conduction, since a part is required to vaporize the water from 

 the skin and lungs. An accurate measurement of the heat production by 

 man therefore required a knowledge of the amount of heat thus vaporized. 

 One of the great difficulties in the numerous forms of calorimeters that have 

 been used heretofore with man is that only that portion of heat measured 

 by direct radiation or conduction has been measured and the difficulties 

 attending the determination of water vaporized have vitiated correspond- 

 ingly the estimates of the heat production. Fortunately, with this apparatus 

 the determinations of water are very exact, and since the amount of water 

 vaporized inside the chamber is known it is possible to compute the heat 

 required to vaporize this water by knowing the heat of vaporization of water. 



Since the earlier reports describing the first form of calorimeters were 

 written, there has appeared a research by one of our former associates, Dr. 

 A. W. Smith * who, recognizing the importance of knowing exactly the 

 heat of vaporization of water at 20, has made this a special object of inves- 

 tigation. When connected with our laboratory a number of experiments 

 were made by Doctors Smith and Benedict in an attempt to determine 

 the heat of vaporization of water directly in a large calorimeter; but for 

 lack of time and pressure of other experimental work it was impossible to 

 complete the investigation. Subsequently Dr. Smith has carried out the 

 experiments with the accuracy of exact physical measurements and has 

 given us a very valuable series of observations. 



Using the method of expressing the heat of vaporization in electrical 

 units, Smith concludes that the heat of vaporization of water between 14 

 and 40 is given by the formula 



L (in joules) =2502.5 -2.43 T 



* Smith: Heat of evaporation of water. Physical Review, vol. 25, p. 145. 

 (1907.) 



