122 A RESPIRATION CALORIMETER. 



the temperature is controlled ; hence heat is not conducted from the 

 chamber by the plug. To prevent the conduction of heat through the 

 metal pipes, each pipe is broken about half way through the plug and 

 the ends are connected by rubber tubing (N, fig. 32). 



The other opening, which is rectangular in shape, is very tightly 

 filled by a wooden box packed with plaster of Paris, through which 

 pass the pipes for the ingoing and the outcoming air. Gain or loss of 

 heat through the box is prevented by the poor conductivity of the box 

 and packing and by the regulation of the temperature of the air sur- 

 rounding the box. To prevent loss by conduction through the metal 

 pipes, each pipe is broken within the box and the ends connected with 

 rubber tubing. (See H, fig. 32.) 



The nature of the opening in the side for the electric cable tube and 

 that in the top for the weighing apparatus is such that no opportunity 

 for loss of heat is afforded. 



GAIN OR Loss OF HEAT THROUGH THE AIR CURRENT. 



As explained elsewhere, part of the heat generated within the cham- 

 ber is carried out as latent heat of water vapor in the outgoing air. So 

 far as the air itself is concerned, however, gain or loss of heat to the 

 chamber due to cooling or heating of the air is prevented by regulating 

 the temperature of the air entering the chamber so that it is exactly 

 the same as that leaving it. 



The difference between the temperature of the incoming and that 

 of the outgoing air is detected by means of iron and German-silver 

 thermal junctions, of much simpler construction than those described 

 above. These are made of double cotton-covered insulated wire, and 

 are bound tightly together in the center like a sheaf of wheat, the bared 

 ends of the junctions being spread out and separated, thus relying on 

 air insulation. These junctions are installed in such a manner that 

 one end is in the incoming and the other in the outgoing air. The 

 apparatus in which they are inserted, designated the vestibule, is 

 shown at T. in figure 33. It consists of a 7.5 cm. copper pipe, with a 

 copper partition. The ends of the thermal junctions are on opposite 

 sides of this partition. The air entering the chamber passes along the 

 under side, and that leaving is in contact with the upper side of the par- 

 tition. The ends of the junctions are connected with a galvanometer, 

 the deflections of which indicate differences in temperature of the air 

 on either side of the partition. 



The incoming air is heated or cooled as necessary, in order that the 

 deflection may be zero. The arrangements for heating or cooling the 

 air are shown in figure 32. 



