HEAT OE WATER, WITH EXPERIMENTS BY A NEW METHOD. 
7 
vary, on the whole, systematically with the temperature of the hot water, and would 
inevitably lead to constant errors. 
It will be observed, on reference to fig. 1, that Ludin’s formula for the mean 
specific heat, when extrapolated, gives results agreeing closely with IIegnault’s 
observations up to 130° C. But this is really without significance, because 
Regnault’s results, as plotted, are not corrected for the probable errors of his 
calorimetric thermometers, and would certainly require to be further reduced. 
Ludin’s curve, if extrapolated to 200° C., would give results about 10 per cent, too 
low, and is obviously of a type which cannot be trusted for extrapolation. It is 
almost inconceivable on any theoretical grounds that the specific heat of water, after 
reaching a maximum at 87° C., should then diminish and increase again. Ludin’s 
method, as already explained, could not be trusted to give certain results with regard 
to the variation of the specific heat near the ends of his range. The experimental 
evidence for the drop in the curve near 100° C. is very weak, and, such as it is, may 
be most readily explained by a slight loss of heat due to evaporation of the nearly 
boiling water on its way to the calorimeter. It would appear almost hopeless to 
obtain reliable results by the method of mixtures with an open calorimeter. 
Regnault’s method, employing a nearly closed calorimeter of large volume, per¬ 
manently connected to the heater by a tube for introducing the water, appears to be 
the only satisfactory means of avoiding the uncertainty of heat loss in transference if 
the ordinary method of mixtures is employed. 
Continuous-Electric Method (Callendar and Barnes). 
The continuous-electric method, in which a steady current of water at any desired 
temperature is heated through a small range of temperature by a steady electric 
current, has the great advantage that it gives directly the actual specific heat over a 
small range at the desired point in place of the mean specific heat over a large range, 
and appears for this reason peculiarly suited for determining the variation of the 
specific heat. The method has been very fully described and discussed in previous 
papers (Callendar, ‘Phil. Trans.,’ A, 1902, voL 199, pp. 55-148; Barnes, Ioc. cit., 
pp. 149-263), but it appears desirable to enumerate briefly its principal features. 
The form of the calorimeter, being merely a fine-bore tube about half a metre long 
with enlargements at either end for the thermometers, gives a very small water 
equivalent and radiating surface, and permits complete enclosure in a hermetically 
sealed vacuum-jacket, which reduces the external heat-loss to a minimum. The 
vacuum-jacket is surrounded by a water-jacket maintained at a steady temperature. 
The water current is brought to the same temperature as the jacket before passing 
the inflow thermometer. The rise of temperature of the water passing through the 
tube is obtained by a single reading on a pair of differential platinum thermometers, 
sensitive to 0°‘0001 C., and probably in all cases accurate to 0°'001 C., thus avoiding 
nearly all the difficulties of mercurial thermometry. The electric heating current 
