HEAT OF EVAPORATION OF WATER. 275 
gas-jets. The water, by passing through the pump, &c., is slightly cooled; thus, the 
work of the regulator is confined to simply supplying the heat lost by convection, 
radiation, &c., and it performs this task admirably. As an illustration, | may mention 
that, in the series of over 50 experiments treated of in this communication, on only one 
oceasion did the temperature of the steel! chamber change by as much as 75° C. 
throughout the duration of an experiment. On the solitary occasion that a change 
amounting to nearly 45° C. was observed, the cause was found in the caking of the 
lime through which the gas was passed on its way to the regulator, and, in 
consequence, the experiment was discarded before working out its results.” 
On account of the use of the differential thermometers (see Section V.) employed 
in the present investigation, it was essential that any changes in the temperature of 
the steel chamber should be measured with greater accuracy than was necessary in 
my previous work, for now such changes influenced the temperature measurements, 
whereas on former occasions they only affected the loss by radiation, ete. An open 
range mercury thermometer placed in mercury in the hole E (Plate 4) gave the 
temperature of the walls surrounding the calorimeter and changes in the mean stem 
temperature had to be guarded against for the reason above given. The small motor 
already referred to now served another purpose. A portion of the water raised by 
the pump, instead of returning to the tank through the silver tube passed into a coil 
of about 20 feet of ‘‘ compo. ” tubing inserted in the tank, was then forced up a glass 
tube surrounding the stem of the thermometer, and passing out at the top, returned 
to the tank. Thus the stem-temperature was kept constant throughout an experi- 
ment, the regularity of the flow being secured by an overflow system. True, the 
water near the top of the glass tube would be slightly cooler than the tank water 
when working at high temperatures, but this was of no consequence, as the chief 
use of the thermometer was to detect differences during an experiment. Two 
thermometers labelled A and II. were used. Although an accuracy of an order of 
zo CU. would have been sufficient in actual elevation, I compared these thermometers 
every 05 C. of their ranges with two different Tonnelot thermometers, standardised 
by the Bureau International des Poids et Mesures, and also with my own platinum 
standard. The results of the separate comparisons (expressed on the nitrogen scale) 
agreed within ‘005° C. | 
The stems of both thermometers were graduated in millimetres. A (range 16° to 
26° C.) having about 27 millims. per 1° C. and II. (range 28° to 53° C.) about 20 
millims. per 1° C. A table was constructed for the whole range of temperature, 
giving the value in degrees C. of (#) each millimetre of these thermometers in terms 
of the air thermometer, and (b) in terms of the millimetres of the “mean bridge wire 
scale” used by me for the determination of differences of temperature. 
These thermometers were observed through a microscope fitted with a micrometer 
scale so divided that it gave 10 divisions to the millimetre. There was no difficulty 
2N2 
