of Mixing- Calorimeter. 249 



Although the principle of this apparatus is extremely 

 simple, it required the utmost skill in workmanship and the 

 greatest attention to minute details to produce anything which 

 would work really satisfactorily*. 



The stirring-apparatus which I use with my single calori- 

 meter consists of a fan-screw with three blades revolving at 

 the bottom of \he liquid, and worked by means of an electro- 

 motor (Chem. Soc. Trans. 1887, p. 293). This apparatus was 

 doubled, so that each half of the mixing- calorimeter should 

 have a separate stirrer, and in each half also one of the most 

 delicate thermometers is suspended, these being tapped con- 

 tinuously on the upper end by the clockwork-tapper (ibid.). 

 Fig. 3 represents the whole apparatus. 



In working this calorimeter it is first of all put in position 

 with the doors closed, and the thermometers and stirrers in 

 their places. The two liquids are then measured or weighed 

 out from flasks into the two compartments, the temperature 

 of each being slightly below that selected for the initial tem- 

 perature of the experiment. The stirrers and tappers are then 

 started, and the temperature of two liquids raised to the 

 required temperature by touching the outside of the calori- 

 meter with some heated object. The temperatures of the two 

 liquids are in this way easily and quickly brought to within 

 0°*02 of each other. 



The thermometers are then read through a telescope at 

 intervals of one minute, till the rate of cooling in each case is 

 perfectly regular : this occurs in a minute or two, and the 

 temperatures of the liquids may then be assumed to be iden- 

 tical. The rate of cooling is then determined for three or 

 four observations ; after which the doors of the calorimeter 

 are opened without stopping the stirrers ; the thermometers 

 are again read, and the readings repeated at intervals of one 

 minute, as before, in order to determine the correction for 

 cooling at the final temperature. 



Although it is not absolutely necessary to have a thermo- 

 meter in each liquid, it is not only advisable as a precautionary 

 measure, but it gives the method a very important advantage 

 over previous methods ; for the heat evolved being measured 

 independently by two different instruments, a single experi- 

 ment has nearly the same value as the mean of two separate 

 ones. 



The temperature, as has been mentioned, may be obtained 

 identical in each division of the vessel before the doors are 



* To the ingenuity and perseverance of W. Peover, of Leigh Street, 

 W.C., I am indebted for success in this work. 



Phil. Mag. S. 5. Vol. 29. No. 178. March 1890. U 



