150 A RESPIRATION CALORIMETER. 



B", and G" and B", of figure 44 ; hence the necessity for the reliability 

 of these connections. With clean mercury and good copper links, the 

 connections are all that could be desired. 



This switch has been in constant use three years and has given 

 excellent satisfaction. Inasmuch as all temperature measurements are 

 relative, not absolute, and the variations in temperature are slight, it 

 can readily be seen that this form of bridge is especially well suited for 

 experimental work of this nature. The sensitiveness is all that could 

 be desired, since with the present adjustment 60 deflections (millimeters) 

 correspond to i C. ; hence readings of 0.01 are readily obtained, an 

 accuracy sufficient at present for experiments with the respiration calo- 

 rimeter. That some form of potentiometer could be used for this work 

 with good results is, of course, not to be doubted, but the compact form 

 of this switch and bridge leaves little to be desired for the purposes for 

 which it was devised. 



DETERMINATION OF THE QUANTITY OF HEAT ELIMINATED. 



It has been shown that heat is regularly carried out of the calorim- 

 eter chamber in two ways partly as latent heat of water vapor in the 

 air current, but chiefly as sensible heat taken up by a current of water 

 circulating in the heat-absorbers. Theoretically, the sum of the two 

 quantities of heat thus removed should equal the total amount elim- 

 inated within the chamber, but in actual practice various corrections 

 must be made to determine the actual quantity of heat. The method 

 of computing the quantity of heat removed in these two ways and the 

 necessary corrections to be applied remain to be described. 



LATENT HEAT OF WATER VAPOR. 



The quantity of heat removed from the chamber in water vapor is 

 found by multiplying the quantity of water collected in the water-ab- 

 sorbers by the factor for latent heat of vaporization of water. In these 

 experiments the factor used is 0.592 calorie per gram, which was de- 

 duced from Regnault's formula, as discussed in detail elsewhere. 1 



It is greatly to be desired that this factor be verified by investigations 

 in which water is vaporized under the conditions that obtain during an 

 experiment with man ; but although considerable preliminary investi- 

 gation of this nature has been made, we have no results that warrant 

 our taking other than the commonly accepted figures of Regnault for 

 this calculation. Presumably the error involved, if any, is not very 

 large. Certainly it is not larger than the probable physiological error 

 in experiments of this type. 



1 U. S. Dept. of Agr., Office of Experiment Stations Bull. 63, p. 57. 



