THERMAL PROPERTIES OF CARBONIC ACID AT LOW TEMPERATURES. 



83 



cent. No calibration 

 copper pipes is necessary. 



The readings, however, had to be made in haste, and the probable error in the weight 

 of CO a might amount to nearly 1 per cent. The balances had to be frequently 

 recalibrated, as the stiffness of the pipes was found to change gradually. Only three 

 of the 17 observations in Table VI., viz., those at 26'1, 14*9, and 8'6, were made 

 while working in this way. 



To get over these defects the following modification was made : One flask was 

 hung on a steelyard, the arm of which was allowed only a very minute movement. 

 When the arm fell it made an electric contact and rang a bell. This arrangement 

 was found to be so sensitive that it would turn with '01 Ib. When the arm fell it 

 was raised again by hanging a weight (usually 1 Ib.) on the flask. When another 

 pound of CO a had passed out of the flask the bell rang again and another weight was 

 hung on, and so on. The increased sensitiveness of this arrangement made it possible 

 to record the rate of flow of CO, accurately at 

 short intervals and to complete the whole test 

 with one flask full or less of CO,, for as small 

 a quantity as 10 Ibs. could be weighed to -^ per 

 No calibration or allowance for the 

 A simple dash-pot 



made of a disc of tin in a vessel of oil got over 

 all difficulties due to vibration without reducing 

 the sensitiveness. 



The calorimeters are tin-plate tanks contain- 

 ing coils of copper pipe and electric heaters; 

 the tanks are lagged on the outside to prevent 

 the inflow of heat. The larger calorimeter, 

 fig. 14, contains two copper coils and the smaller 

 one a single coil. The coils can be connected in 

 different ways for the different series of tests. 

 Calcium chloride brine was used to fill the tanks 

 for the first experiments, but was replaced 

 later by methylated spirits, which answered 

 much better. Special care was taken in the 

 ilfsign of the calorimeters to provide a definite 

 path for the circulation of the liquid, which was maintained by a screw propeller 

 driven by an electric motor. The large calorimeter was originally lagged with 

 2 inches of slag wool ; this was found to be insufficient and 2 inches of felt were 

 added, covered by varnished calico to keep out the moisture. A wooden top was 

 fitted, covered by felt and calico. The small calorimeter, which was completed after 

 experience had been gained with the larger one, was wrapped in 2 inches cotton 

 wool surrounded by about 2 inches slag wool, all contained in a wooden box. The 

 cover was formed of 3 inches of wood. Several sorts of heater were tried and failed ; 



M 2 



Fig. 14. 



