PROF. II. L. CxVLLENDAR ON THE VARIATION OF THE SPECIFIC 
which was proposed at the British Association meeting in 1899 as an easily 
reproducible scale of reference for the experimentalist. The correction of the 
practical scale, as above defined, to the absolute scale is so small and uncertain, and 
has so often been applied incorrectly, that its application appears more likely to lead 
to confusion than to improved agreement in experimental work. 
The specific heat of water at 15° C. has often been adopted as the standard. It 
exceeds that at 20° 0. by little more than 1 in 1,000. The reduction from 15° C. to 
20° C. can be efiected with comparative certainty, but does not materially affect the 
question of the variation of the specific heat, since all the values are altered nearly in 
the same proportion, and few results are accurate to 1 in 1,000. 
Begnault’s Experiments, 100° C. to 200° C. 
Begnault operated by mixing 10 litres of water from a boiler at various tempera¬ 
tures between 107° C. and 187° C. with 100 litres of water in a calorimeter at the 
atmospheric temperature. His ol)servations gave directly the mean specific heak^ of 
water from the temperature and pressure of the l)oiler to the final temperature of the 
calorimeter at atmospheric pressure in terms of the mean specific heat between the 
initial and final temperatures of the calorimeter. The results did not give any direct 
eA'idence with regard to the variation of specific heat between 0° G. and 100° C., but 
were fairly consistent over the range 100° C. to 200° C. with the assumption of his 
well-known parabolic formula 
s,* = 1-f 0-002(^/100)+ 0-003 (^/100)2.(2) 
for the mean sj^ecific heat between 0° C. and t° C. 
Howland’s discovery, in 1879, that the specific heat of water diminished by about 
1 per cent, between 0° G. and 30° G., showed that Begnault’s formula could not 
possibly be correct at low temperatures, and necessitated a recalculation of his 
thermometer, and assuming the absolute expansion of mercury from the observations of Callendar and 
Moss (‘Phil. Trans. Roy. Soc.,’ A, vol. 211, pp. 1-32) give a value 1 -00 x between 0° C. and 100° C., 
increasing to 1 "50 x 10“'^ between 0° C. and 184° C. If this should be correct, the result of HOLBORN and 
Henning would require to be raised by 0°’10 C., giving 444°'61 C. on the absolute scale. It is hoped 
that experiments now in progress by N. Eumorfopoulos by the constant-pressure method, with a quartz- 
glass thermometer of the Callendar type, will throw further light on this important point. It would 
appear in any case that formula (1) gives a better apjiroximation to the absolute scale of temperature than 
has previously been supjjosed. It has been possible for the last twenty years to obtain jilatinum for 
thermometric purposes of the same unvarying degree of purity, giving a temperature coefficient approxi¬ 
mating to O'00390, and a practically constant difference-coefficient. It seems, therefore, preferable to 
eliminate errors of observation of the sulphur boiling-point, and differences of opinion as to its absolute 
value, by assuming a standard value 1'50x10“^ for the difference-coefficient in the definition of "the 
practical scale. 
* More accurately, the change of total heat E+jiv. 
