THE CALORIMETER SYSTEM AND MEASUREMENT OP HEAT. 121 



as to necessitate the placing of the steam-pipes near the ceiling of the 

 calorimeter laboratory, and consequently the upper layer of air is very 

 much warmer than that near the bottom. In fact, a temperature dif- 

 ference of from 6 to 10 is not at all uncommon. By means of two 

 rotating ceiling fans, however, it is possible to distribute the warm air 

 and thus equalize the temperature in the laboratory to a considerable 

 degree, the blades of the fan being so adjusted as to force the warm air 

 downward. Cooling the air is effected by opening windows at different 

 parts of the laboratory. By these means it has been found possible to 

 regulate the temperature opposite the window of the calorimeter with 

 considerable accuracy. 



Another mercury thermometer is hung on the outside of the rear of 

 the calorimeter, near the food aperture. This likewise serves as a guide 

 whereby the temperature of the air in which it is hung may be kept not 

 far from 20, i. e., that of the calorimeter chamber. Any great inter- 

 change of heat through the food aperture is thus prevented. Further- 

 more, the tube of the food aperture is surrounded for part of its length 

 by the air in the inner air-space and through another part of its length 

 by the air in the outer air-space, and the temperature of these portions of 

 the tube is of course that of the air in the spaces, which is controlled, as 

 explained above, in accordance with that of the chamber. Interchange 

 of heat between the metal of the tube and the orifice in the metal walls 

 through which it passes is prevented by the rubber tube filled with 

 compressed air (D, in fig. 8) by which they are separated. 



The window opening in front of the calorimeter, and through it the 

 food aperture in the rear end, may be plainly seen in figure 30. Adja- 

 cent to the window in the front end are three other openings, at the 

 places where the dark objects are seen projecting from the metal wall. 

 These are shown in detail in the perspective view in figure 33. 



The smallest of these openings, near the lower corner of the window, 

 is for the passage of an iron rod, which is the axis of the device for 

 raising and lowering the shields to the heat-absorbers, described beyond. 

 The rod is fitted very closely into a sleeve in the copper wall through 

 which it passes. The metal of the rod is of course a good conductor 

 of heat, but, as it passes through the two outer air-spaces, its temper- 

 ature is controlled by that of the air surrounding it, and thus little 

 opportunity is afforded for the passage of heat through it. 



The larger circular opening to the right of the rod (fig. 32) is very 

 tightly closed by a wooden plug through which pass the two pipes for 

 conducting the water used to bring away heat from the interior of the 

 chamber, as explained later. The wood itself is a very poor heat-con- 

 ductor, and furthermore it is surrounded by the two air-spaces in which 



