Ox THE MECHANICAL EQUIVALENT OF HEAT 389 



twenty per cent in specific heat, and in a very irregular manner, first 

 decreasing, then increasing, and again decreasing. But soon after an- 

 other paper appeared, showing that the results of the previous experi- 

 ments were entirely erroneous. 



The new experiments, which extended up to 13 C., seemed to give 

 an increase of specific heat up to about 6, after which there was appar- 

 ently a decrease. It is to be noted that Geissler's thermometers were 

 used, which I have found to depart more than any other from the air 

 thermometer. 



But as the range of temperature is very small, the reduction to the 

 air thermometer will not affect the results very much, though it will 

 somewhat decrease the apparent change of specific heat. 



In the Journal de Physique for November, 1878, there is a notice of 

 some experiments of M. von Miinchausen on the specific heat of water. 

 The method was that of mixture in an open vessel, where evaporation 

 might interfere very much with the experiment. No reference is made 

 to the thermometer, but it seems not improbable that it was one from 

 Geissler; in which case the error would be very great, as the range was 

 large, and reached even up to 70 C. The error of the Geissler would 

 be in the direction of making the specific heat increase more rapidly 

 than it should. The formula he gives for the specific heat of water at 

 the temperature t is 



1 -f -000302 i. 



Assuming that the thermometer was from Geissler, the formula, re- 

 duced to the air thermometer, would become approximately 



1 -00009 t+ -0000015 t 2 . 



Had the thermometer been similar to that of Kecknagel, it would 

 have been 1 -f -000045 t -f -000001 t 2 . 



It is to be noted that the first formula would actually give a decrease 

 of specific heat at first, and then an increase. 



As all these results vary so very much from each other, we can 

 hardly say that we know anything about the specific heat of water 

 between and 100, though Kegnault's results above that temperature 

 are probably very nearly correct. 



It seems to me probable that my results with the mechanical equiv- 

 alent apparatus give the variation of the specific heat of water with 

 considerable accuracy; indeed, far surpassing any results which we 

 can obtain by the method of mixture. It is a curious result of those 

 experiments, that at low temperatures, or up to about 30 C., the spe- 



