O CALORIFIC POWER OF FUELS. 



The metastatic thermometer is a differential thermometer 

 with a variable scale. At will, a certain quantity of mercury 

 flows into the bulb. By this means we raise or lower the 

 degrees for which it may be used. Suppose an ordinary 

 thermometer graduated from o to 10, and left open at 

 the top at the loth degree. If we wish to use it between 12 

 and 14, heat it to 14, and a portion of mercury correspond- 

 ing to 4 escapes. Now, instead of showing a difference of 10 

 between o and 10, it will show this difference between 4^ 

 and 8, the original o having descended to 4. It will be 

 similar for temperatures of 10, 20, or 30, as desired. By 

 closing the thermometer at the top instead of leaving it open, 

 and blowing a bulb in the upper portion as overflow, the 

 conditions will remain the same. The thermometer has now 

 become metastatic. These thermometers are made by Baudin 

 of Paris, from whom full directions for use and corrections can 

 be obtained. 



With all thermometers it is essential that the glass of the 

 bulb should be rather thin, or the thermometer will be " too 

 slow." The slightest difference in temperature must be 

 shown immediately by a movement of the mercurial column. 

 To test for sensibility, read the height of the column and then 

 place the hand on the bulb. If sufficiently sensitive the mer- 

 cury will descend quickly from the expansion of the glass and 

 afterwards rise. In thermometers divided to T ^ this move- 

 ment should be immediate, and over several hundredths. 



In ordinary calorimetric experiments the correction due to 

 length of the mercury column flowing out of the bulb may 

 be neglected for several reasons; the experiments should be 

 made in a room where the temperature is nearly the same as 

 that of the calorimetric bath, such correction would be of 

 very little consequence for a slight change of temperature, 

 and the experimenter should plunge the thermometer into the 

 bath as deep as is necessary to take the reading at the level 

 of the eye. 



