of the University of Pennsylvania. 69 



IX. 



A METASTATIC HEAT REGULATOR. 



THE instrument about to be described is adaptefl 

 to maintain a constant temperature within any water- 

 or air-chamber heated by gas, the degree of temper- 

 ature thus maintained being adjustable at will. 



Reference to the illustration shows an air-ther- 

 mometer so modified that the rise of mercury in the 

 limb B will cut off the gas-supply which passes 

 through its bifurcated extremity. A second modifi- 

 cation lies in the accurately-fitting glass stop-cock 

 D, connected with the air-chamber A. By means 

 of this stop-cock the tension of the air within the 

 chamber, and consequently the height of the mer- 

 cury in the tube B, is readily adjustable. It is evi- 

 dent that when the mercury is forced high up in B, 

 a relatively slight increase in the temperature of the 

 surrounding medium will be sufficient to so expand 

 the air in A as to force the column of mercury to 

 the point of shut-off. On the other hand, a far 

 higher temperature will be needed to effect the 



