THE CALORIMETER. 25 



and difficulty was constantly experienced. An attempt was next made to 

 substitute hard-rubber tubing for the glass tube, but this did not prove to 

 be an efficient insulator. More recently we have used with perfect success 

 a special form of vacuum-jacketed glass tube, which gives the most satis- 

 factory insulation. However, this system of insulation is impracticable 

 when electric-resistance thermometers are used for recording the water- 

 temperature differences and can be used only when mercurial thermometers 

 exclusively are employed. The electric-resistance thermometers are con- 

 structed in such a way, however, as to make negligible any inequalities in 

 the passage of heat through the hard-rubber casing. This will be seen in 

 the discussion of these thermometers. 



Measuring the water. As the water leaves the respiration chamber it 

 passes through a valve which allows it to be deflected either into the drain 

 during the preliminary period, or into a small can where the measure- 

 ments of the rate of flow can readily be made, or into a large tank (G, fig. 

 14) where the water is weighed. The measurement of the water is made by 

 weight rather than by volume, as it has been found that the weighing may 

 be carried out with great accuracy. The tank, a galvanized-iron ash-can, 

 is provided with a conical top, through an opening in which a funnel is 

 placed. The diagram shows the water leaving the calorimeter and entering 

 the meter through this funnel, but in practice it is adjusted to enter through 

 an opening on the side of the meter. After the valve f is tightly closed the 

 empty can is weighed. 



When the experiment proper begins the water-current is deflected so as 

 to run into this can and at the end of an hour the water is deflected into 

 a small can used for measuring the rate of flow. While it is running into 

 this can, the large can G- is weighed on platform scales to within 10 grams. 

 After weighing, the water is again deflected into the large can and that 

 collected in the small measuring can is poured into G through the funnel. 

 The can holds about 100 liters of water and consequently from 3 to 8 one- 

 hour periods, depending upon the rate of flow, can be continued without 

 emptying the meter. When it is desired to empty the meter at the end of 

 the period, the water is allowed to flow into the small can, and after weigh- 

 ing G, the valve / is opened. About 4 minutes are required to empty the 

 large can. After this the valve is again closed, the empty can weighed, and 

 the water in the small measuring-can poured into the large can G through 

 the funnel. The scales used are the so-called silk scales and are listed by 

 the manufacturers to weigh 150 kilograms. This form of scales was formerly 

 used in weighing the man inside the chamber.* 



* W. O. Atwater and F. G. Benedict: A respiration calorimeter with appliances 

 for the direct determination of oxygen. Carnegie Institution of Washington 

 Publication No. 42, p. 158. (1905.) 



