22 
PROF. H. L. CALLENDAR ON THE VARIATION OF THE SPECIFIC 
Ludin's curve from 65° C. to 100° C. is represented in three pieces, the curve being 
shifted downwards through 0'200 when it reaches the upper limit of the diagram. 
Rowland’s observations on the mechanical equivalent from 5° C. to 35° C., if plotted 
on the same scale, would agree to 0'003 throughout his range with formula (lO), the 
full curve. His experiments do not, as generally stated, afiPord any conclusive 
evidence of a minimum at 30° C. in the specific heat. Rowland himself considered 
that, owing to the increasing magnitude and uncertainty of the radiation correction 
beyond 30° C., “there might be a small error in the direction of making the 
mechanical equivalent too great at that point (30° C.), and the specific heat might 
keep on decreasing to even 40° C.” The discrepancy from Ludin’s curve in this 
region is less than 1 in 1,000 of the total heat, and is within the limits of error of 
Ludin’s experiments. Formula (6) gives a minimum at 37°’63 C. Ludin’s formula 
gives a minimum at 25°'22 C., which cannot be reconciled with Rowland’s obser¬ 
vations. 
The point at which the specific heat in absolute units can be most accurately 
infei’red from Rowland’s experiments is 20° C., at the middle of his temperature 
range. The value at this point is probably within 1 in 2,000 of 4T80 joules per 
gr.-deg. C. on the scale defined by formula (l). Rowland himself gave the value 4T79 
on the air scale; Day’s reduction of Rowland’s thermometers gave 4’181 on the 
hydrogen scale. The results of Reynolds and Moorby, when corrected to cover the 
range 0° C. to 100° C., give 4T84 joules for the mean calorie. Combining these two 
results we find 4T84/4T80, = I'OOlO, for the ratio of the mean calorie to the calorie 
at 20° C. My formula (lO) would make the ratio I’OOIG, which is fairly good agree¬ 
ment. But Ludin’s formula gives the ratio 1'0063, showing a discrepancy of O'53 
per cent., which would appear to be beyond the possible limits of error of Reynolds 
and Moorby’s experiments, since the extreme variations of their results did not 
exceed 0'36 per cent., and the mean would probably be correct to 1 in 1,000. 
The evidence to be deduced from the observations of W. R. Bouseield and 
W. E. Bouseield {loc. cit.) with regard to the variation of the total heat rests on 
their “point-to-point” experiments over the following ranges :— 
Range of temperature . 0-13°C. 13-27°C. 27—40°C. 40-55°C. 55-73°C. 55-80°C. 
Mean value of J . 
4'1937 
4'1752 
4'1756 
4'1935 4'2024 
4'2056 
dh deduced 
0'058 
0'058 
0'059 
0'124 0'242 
0'306 
dll Ludin . . . . 
0'057 ’ 
0'059 
0'064 
0'119 0'285 
0'371 
The values of the 
variation dh 
at the points 13° C., 27° C., 40° C., 55° 
C., 73° C., 
and 80° C. are deduced from Messrs. Bouseield’s observations by taking their value 
4'1752 for the unit at 20° C. and adding up their values of the total heat for the 
separate ranges. Up to 55° C. they agree very closely with Ludin’s curve, as 
indicated by the crosses in fig. 6. Their values at 73° C. and 80° C. are somewhat 
