2OO Historical Review of Experiments 



rising and falling of the mercury, I changed my method 

 of operation, and no longer observed the time neces- 

 sary for the instrument to grow warm, but that neces- 

 sary for it to grow cold. 



When, therefore, my apparatus, plunged in boiling 

 water, had acquired such a temperature that the mercury 

 had reached 77 of Reaumur's scale, I took it out of 

 the boiling water and held it in the air, over the large 

 vessel filled with pounded ice and water, ready to plunge 

 it into this cooling mixture the very moment that the 

 mercury had fallen to 75. 



As soon as the mercury had reached this division of 

 the scale, I plunged my apparatus immediately into the 

 cooling mixture, and holding at the same time at my 

 ear a watch which beat half-seconds (which I carefully 

 counted), I waited for the moment when the mercury 

 had fallen to 70. I then noted and recorded the time 

 elapsed, and in the same way observed the time when 

 the mercury had fallen to 60, and thus proceeded, 

 noting every ten degrees, until the apparatus had cooled 

 to the temperature of 10. 



Sometimes the apparatus cooled to such an extent 

 that the mercury in the thermometer stood at o ; this, 

 however, took up much time, and was attended with no 

 particular advantage, as the determination of t|ie times 

 taken up in cooling from 70 to 16 was quite sufficient 

 for calculating the conducting power of every sort of 

 covering ; on this account I generally ended the experi- 

 ment when the mercury had just passed the 10 mark 

 on the scale. 



During the time of cooling the apparatus in ice and 

 water, I moved it about in the mixture very slowly 

 and constantly from one place to another ; moreover, I 



