440 HENRY A. EOWLAXD 



long, extending through a cork in the top of the calorimeter. The top 

 of the safety-tube was nearly closed by a cork to prevent evaporation. 

 Had the tube been shorter, water would have been forced out, as well 

 as air. 



2. Conduction along stem from outside to thermometer bulb. To 

 avoid this, not only was the bulb immersed, but also quite a length of 

 stem. As this portion of the stem, as also the bulb, was surrounded 

 by water in violent motion, there could have been no large error from 

 this source. The immersed stem to the top of the bulb was generally 

 about 5 cm. or more, and the stem only about -8 cm. in diameter. 



3. The thermometer is never at the temperature of the water, be- 

 cause the latter is constantly rising; but we do not assume that it is 

 so in the experiment. We only assume that it lags behind the water 

 to the same amount at all parts of the experiment, and this is doubt- 

 less true. 



To see if the amount was appreciable, I suddenly threw the apparatus 

 out of gear, thus stopping it. The temperature was observed to con- 

 tinue rising about 0-02 C. Allowing 0-01 for the rise due to motion 

 after the word "Stop" was given, we have about 0-01C. as the 

 amount the thermometer lagged behind the water. 



4. Evaporation. A possible source of error exists in the cooling of 

 the calorimeter by evaporation of water leaking out from it. 



The water was always weighed before and after the experiment in 

 a balance giving -i. gramme with accuracy. The normal amount of 

 loss from removal of thermometer, wet corks, &c., was about 1 gramme. 

 The calorimeter was perfectly tight, and had no leakage at any point 

 in its normal state. Once or twice the screws of the stuffing-box 

 worked loose, but these experiments were rejected. 



The evaporation of 1 gramme of water requires about 600 heat units, 

 which is sufficient to depress the temperature of the calorimeter about 

 0-07 C. As the only point at which evaporation could take place was 

 through a hole less than 1 mm. diameter in the safety-tube, I think it 

 is reasonable to assume that the error from this source is inappreciable. 

 But to be doubly certain, I observed the time which drops of water of 

 known weight and area, placed on the warm calorimeter, took to dry. 

 From these experiments it was evident that it would require a consid- 

 erable area of wet surface to produce an appreciable effect. This wet 

 surface never existed unless the calorimeter was wet by dew deposited 

 on the cool surface. To guard against this error, the calorimeter was 

 never cooled so low that dew formed; it was carefully rubbed with a 



