o36 Prof. Spencer U. Pickering on Delicate 



The difference between the first and remaining intervals 

 being — '14 and — *18 in the two experiments. Thus which- 

 ever be the direction of the motion of the mercury it will 

 continue in that direction, giving correct results, but, if the 

 direction be changed, an interval elapses before the motion 

 becomes apparent, and a considerable error will be introduced. 



The above are but typical experiments out of a large number 

 performed, and all of which gave the same results ; indeed, 

 every determination of the heat of dissolution of a salt afforded 

 a fresh exemplification of the fact. 



Of course the alteration in the size of the bulb will invali- 

 date any experiment unless its effect be counteracted. This 

 was always done in every experiment by taking care that, if 

 a rate of cooling was to be measured after the salt had been 

 dissolved, the liquid should be cooling also before its intro- 

 duction, and vice versa: the motion of the mercury, and 

 therefore the shape of the bulb, being the same at the begin- 

 ning and end of the experiment, no error would occur. In 

 such an experiment, where for instance a rate of cooling ob- 

 tained, and the salt dissolved with absorption of heat, no period 

 of inertia in the column would occur ; but if the salt dissolved 

 with evolution of heat, two such periods would occur, and, 

 though this would not affect the accuracy of the results, a 

 certain time would elapse before the apparent rate of cooling 

 became constant, extending often to three intervals of 3 or 5 

 minutes each (for the whole of these intervals cannot be spent 

 in tapping the thermometer as in the experiments quoted 

 above, but are chiefly occupied in reproducing the actions 

 performed in effecting the dissolution of the salt), thus creating 

 a serious increase in the duration of an experiment. This 

 was obviated by removing the thermometer from the calori- 

 meter, touching the bulb with some warm object, when, on 

 replacing it, the column fell to the temperature of the calori- 

 meter, and proceeded to give the rate of cooling at once ; where 

 a rate of heating was to be determined, the thermometer was 

 cooled (by evaporation from the surface) instead of heated. 

 As a few millim. rise or fall are sufficient for the purpose, the 

 heating or cooling of the contents of the calorimeter in such 

 an operation is quite negligible *. 



The change of size in the bulb, as deduced from the expe- 

 riments already quoted, is, perhaps, somewhat exaggerated, 

 owing to an under-estimation at the time of the amount of 



* The water- equivalent of the immersed portion of this thermometer 

 was 2-09 grams, that of the calorimeter and contents about 610 grams. 

 A heating of the former sufficient to produce a rise of 5 mm. -would 

 raise the temperature of the latter only O'OCOl (J. 



