THERMAL PROPERTIES OF CARBONIC ACID AT LOW TEMPERATURES. 89 



(iv.) Change of Temperature of the. Hath during a Test. The temperatures of the 

 bath at the beginning and end of a test were accurately determined by taking several 

 observations of temperature at short intervals before and after the actual moment of 

 the start or finish and plotting them. The temperature at the actual moment 

 of start or finish was then road from the graph. Any change of temperature of the 

 bath showed that the electric heat supplied had been slightly too much or too little. 

 The balance of heat (excess or shortage) is simply the temperature rise or fall 

 multiplied by the water equivalent of the calorimeter. The water equivalent was 

 determined by a simple heating experiment when the bath was at approximately the 

 atmospheric temperature so that radiation could be neglected. The results obtained 

 in this way are not quite accurate since the heat capacity of the lagging varies with 



the rate of heating. 



Radiation. 



A number of experiments were made to determine the rate at which heat 

 entered the calorimeters from the surrounding air before concordant results were 

 obtained. In the end good results were arrived at and the radiation was found to be 

 proportional to the difference of temperature between the calorimeter and the 

 atmosphere. The rate for the large calorimeter was '83 C. per hour with a 

 temperature difference of 40 C. The water equivalent being 97 '5 this radiation 

 corresponds to *83x97'5 = 81 thermal units per hour for 40 C. difference of tempe- 

 rature. The small calorimeter rate was 1'92 C. per hour or I'92xl5 = 28'8 thermal 

 units per hour for 40 C. difference of temperature. These figures include the 

 mechanical work put in by the stirrer motors, which in the small calorimeter was 

 measured and found to be about 2 '2 thermal units per hour. 



Conduction along the Pipes. 



The calorimeter coils, and most of the connecting pipes, were made of copper, 

 inch external diameter, J inch internal diameter. Such a pipe would conduct about 

 11 thermal units per hour with a temperature gradient of 40 C. per foot. To 

 minimise conduction, the small calorimeter had a piece of thin-walled steel pipe" 

 inserted in both the ingoing and outgoing connections which would conduct heat at 

 about one-sixth of the above rate. All the pipes were well lagged. When the 

 apparatus is working it can easily be shown that there can be only a very small 

 temperature gradient beyond the thermo-junctions so that no appreciable conduction 

 can take place. Temperature measurements along the pipes confirmed this. When 

 the vapour is not circulating the conditions are not quite the same and a small 

 amount of conduction takes place. This conduction makes the apparent radiation 

 rather too large, so that the corrections applied for radiation are a little too large. 

 An approximate estimate shows that the error introduced is not greater than 

 '3 thermal units in the value of L at the lowest temperature and that it will not be 

 appreciable above 30 C. 



VOL. ccxni. A. N 



