18 



CALORIMETERS FOR STUDYING- RESPIRATORY EXCHANGE, ETC. 



amount of wire used in any one circuit is equal to a resistance of approxi- 

 mately 92 ohms. This method of warming the air-space leaves very little 

 to be desired. It can be instantaneously applied and can be regulated with 

 the greatest ease and with the greatest degree of refinement. 



If, on the other hand, it becomes necessary to cool the air-space next to 

 the zinc and in turn cool the zinc, we must resort to the use of cold water, 

 which is allowed to flow through the pipe C suspended in the air-space 

 between the zinc and hair-felt at approximately the same distance as is the 

 heating wire. The support of these pipes is accomplished by placing them 

 in brass hangers G, soldered to the zinc and provided with an opening in 

 which the pipe rests. 



In the early experimenting, it was found impracticable to use piping of 

 very small size, as otherwise stoppage as a result of sediment could easily 

 occur. The pipe found best adapted to the purpose was the so-called 

 standard one-eighth inch brass pipe with an actual internal diameter of 



7 millimeters. The opening of a valve 

 allowed cold water to flow through this pipe 

 and the considerable mass of water passing 

 through produced a very noticeable cooling 

 effect. In the attempt to minimize the 

 cooling effect of the mass of water remain- 

 ing in the pipe, provision was made to allow 

 water to drain out of this pipe a few mo- 

 ments after the valve was closed by a system 

 of check-valves. In building the new appa- 

 ratus, use was made of the compressed-air 

 service in the laboratory to remove the large 

 mass of cold water in the pipe. As soon as 

 the water-valve was closed and the air-cock 

 opened, the compressed air blew all of the 

 water out of the tube. 

 The best results have been obtained, however, with an entirely new prin- 

 ciple, namely, a few drops of water are continually allowed to pass into the 

 pipe, together with a steady stream of compressed air. This cold water is 

 forcibly blown through the pipe, thus cooling to an amount regulated by 

 the amount of water admitted. Furthermore, the relatively dry air evap- 

 orates some of the water, thereby producing a somewhat greater cooling 

 effect. By adjusting the flow of water through the pipe a continuous cool- 

 ing effect of mild degree may be obtained. While formerly the air in the 

 space next the zinc wall was either cooled or heated alternately by opening 

 the water-valve or by passing a current through the heating coil, at present 

 it is found much more advantageous to allow a slow flow of air and water 

 through the pipes continuously, thus having the air-space normally some- 



Fig. 13. — Detail of drop-sight feed-valve 

 and arrangement of outside cooling 

 circuit. The water enters at A, and 

 the flow is regulated by the needle- 

 valve at left-hand side. Rate of flow 

 can be seen at end of exit tube just 

 above the union. The water flows 

 out at C and compressed air is ad- 

 mitted at B, regulated by the pet- 

 cock. 



