492 



KEPORT — 1888. 



nitrogen, hydric chloride, carbon monoxide, nitric oxide ; bodies wbich, 

 as gases, sbow a remarkable conformity to Boyle's and Gay-Lussac's 

 laws in respect of the variation of volume with compression and dilata- 

 tion ; for such bodies the heat-capacities of equal volumes at constant 

 pressure were found to be nearly the same, the smallest value being 

 0'2333 for hydric chloride, and the largest 0'2438 for nitrogen. 



If we can assume for each of these gases that a volume of it expe- 

 riences no change of temperature as a consequence of expansion into a 

 vacuum — that therefore no internal work is employed in the act of ex- 

 panding — the specific heat at constant volume can be calculated from that a.t 

 constant pressure in each case by calculating the thermal equivalent of 

 the external work done by the expansion of 1 gram of the gas from 0° to 

 1° against the pressure of the atmosphere and subtracting this result from 

 the specific heat at constant jjressure ; the remainder is the specific heat 

 at constant volume. 



Trom this can easily be deduced the lieat-capacity of a litre of the gas 

 at constant volume by multiplying the specific heat of (a gram of) the gas 

 by the gram-weight of a litre of it ; the result so obtained as the heat- 

 capacity of a litre of air at constant volume is 0'218 ; and the difference 

 would be the same for each of the gases mentioned if its changes of 

 volume by compression and dilatation accurately followed Boyle's and Gay- 

 Lussac's laws, and if the internal energy suffered absolutely no change. 

 The actual result in each case is only a23proximate and subject to 

 varying amounts of error, due to the greater or less deviation of the above 

 suppositions from accuracy within the conditions of the experiments. 



The approximate specific heat of air at constant volume is •1685 ; at 

 constant pressure '2375, the difference '0690 being the heat equivalent 



gram 



of 



air 



against 



atmoswheric 



of the external work done by one 

 pressure under the conditions. 



Again the specific heat of hydrogen at constant pressure being 3"408, 

 that at constant volume is 2'415, the difference being "993 which is the 

 equivalent of the work done by the expansion of ll'lC litres (the volume 

 of 1 gram of hydrogen at 0° and 7Q cm.) against the atmospheric pressure 

 7Q cm. during the rise of temperature of this volume of hydrogen from 

 0° to 1°. 



The heat-capacity of 2 grams (or 22'32 litres under the same con- 

 ditions) of hydrogen is therefore 6'8]6 at constant pressure, 4'830 at 

 constant volume, the difference being 2 x ■993=1"98G. 



The appended table gives the specific heats at constant pressure and 

 at constant volume of 22'32 litres of a number of gases referred to 0° 

 and 76 cm., calculating that at constant volume from that at constant 

 pressure as above. 



At constant volume. 



In the above table it will be noticed that the first six gases have 



