364 PROCEEDINGS OF THE AMERICAN ACADEMY. 



mined for any temperature at will. The manometer also served as a 

 criterion of constancy of temperature. 



Description of Apparatus. 



Heating Apparatus and Thermostat. 



The heating is done electrically and the temperature is controlled by 

 a self-regulating thermostat. As a large amount of work has been 

 planned requiring an automatically controlled thermostat working over 

 a wide range of temperature, it was decided to build it once for all ; and, 

 though for a large amount of work a simpler controller might be used, 

 the ease of manipulation and certainty of operation repaid for the extra 

 trouble in building. 



The thermostat is a modification of the one used by Griffiths * in his 

 work on " The Value of the Mechanical Equivalent of Heat." 



It consists of a cast-iron box A (Figure 2), the walls of which are 

 hollow and filled with mercury. The expansion of the mercury closes 

 the relay circuit, which controls the heating current. The construction of 

 this box was attended with serious difficulty because, on account of the 

 openings for fan shafts, windows, and experimental tubes, it was deemed 

 best to make the box in one casting. The walls are hollow except where 

 the openings occur, where they are solid for a space of about 1 cm. 

 about the openings. I am indebted to Mr. L. S. Burbank for the 

 method of making the patterns for this casting. The hollow walls and 

 the interior of the box itself were cored, the pattern being one solid piece. 



The top of the box was planed smooth, and then ground by rubbing 

 on a large sheet of plate glass covered with powdered emery and oil. 

 The cover of the mercury chamber was also planed and ground, and 

 screwed on tightly with just enough silicate of soda to ensure perfect 

 tightness. Finally the removable cover was fitted on. Then the holes 

 for the windows and fan shafts in the sides and for the experimental 

 tubes in the bottom were drilled. These holes were of course bored 

 through the solid places in the walls. The dimensions of the box are 

 15x15x15 cm. outside. The walls are 6 cm. thick, the hollow space 

 being the same size. Both the cover to the mercury chamber and the 

 removable cover are .6 cm. thick. The mercury chamber contains 420 c.c. 

 There are two openings in the cover to this chamber, one for the 

 overflow to the regulator and the other for convenience in filling, which 



* Griffiths, Phil. Trans., 184, 361 (1893). 



