CAEBON DIOXIDE AT DIFFERENT TEMPERATURES. 233 



nearly steady. A was then opened and B shut off, so that the final readings were 

 taken with the pure gas from A passing through the apparatus. The process could 

 be repeated until A was empty, by which time B was half empty. A was then 

 replaced by a full bottle, and B was used for supplying the gas while the measurements 

 were being taken, and so on. 



This method enables one to make use of all the gas from the bottles, and yet to use 

 only pure CO 2 when taking the actual measurements. As a matter of fact, the 

 presence of a small quantity of air in the C0 2 only produces a very small effect on the 

 measured value of the specific heat. The volumes of air and CO 2 which flow through 

 the fine tubes for a given pressure difference between their ends are practically the 

 same as may be seen from the calibration tables ; further, the thermal capacities of 

 equal volumes of air and CO 2 are in the ratio 1 to 077. Hence, if as much as 

 1 per cent, of C0 2 were replaced by an equal volume of air, which would leave the 

 apparent value of Q unchanged, the thermal capacity would only be reduced by 

 0'23 per cent. It is probable that in no case was there more than 0'4 per cent, of air 

 in the C0 2 when the measurements were taken, as the bottles had always been 

 running for some time before the temperature conditions had become steady, so that 

 the effect of the presence of air in the CO 2 never probably amounted to more than 

 one or two parts per thousand on the energy supplied in any experiment. 



Before commencing the experiments it was necessary to "get rid of all the air in the 

 different parts of the apparatus ; this wa done by exhausting the air from the 

 different parts and allowing CO 2 to take its place, the operation being repeated 

 several times. 



(25) The Calibration of the Fine Tubes. The method adopted was similar to that 

 described for air. For filling the reservoir with C0 2 a combined suction and com- 

 pression pump was used, the gas being drawn through the sulphuric acid rocker and 

 forced into the reservoir. In order to minimise the time taken for the oil in the oil 

 gauge to settle down in an experiment, it was previously blown up to a reading 

 a little above that at which it was finally expected to settle in the experiment, and 

 the glass taps were turned so as to keep it there while the oil on the sides of the 

 gauge settled down. After about twenty minutes the current of gas was started, 

 and the actual experiment was commenced, the taps being turned so that the gauge 

 was under the influence of the pressure difference set up by the gas flowing through 

 the fine tubes. 



The following abridged table represents the results for the calibration and 

 corresponds to the table given on p. 221 for air. The complete records of the calibration 

 are preserved in the archives. The correction of the value of PV to the value of P V . 

 which it would have had if the gas had obeyed BOYLE'S law, was obtained as in the 

 case of air by an expression of the type P V = PV{(P 76)m + l}, where m is a 

 quantity which may be determined from REGNAULT'S tables. In the case of CO 2 

 there was an appreciable difference between the values of m for the high and low 



VOL. COX. A. 2 H 



