234 Mr. AY. Makower on a Determination of the 



half an atmosphere, so that the steam in the inner vessel was 

 superheated about 10° 0. 



The pressure in the jacket was kept constant by means o£ 

 an automatic gas-regulator devised by Callendar, which con- 

 trolled the supply of coal-gas reaching the burner employed 

 for heating the boiler which generated the steam. It was 

 found that by this device the pressure could be kept constant 

 to 1 mm. of mercury. To keep the temperature as constant 

 as possible the whole vessel was packed in cotton-wool. 



To prevent the condensation of steam in the tube E a 

 small metal tap T was attached close to the vessel; this 

 tap was not opened until the pressure in the vessel had 

 become constant. By pumping in air the pressure in the 

 tube E was raised slightly above that of the steam in the 

 experimental vessel, so that on opening the tap T a small 

 quantity of air passed into the vessel preventing steam from 

 passing into the tube E and condensing there. In order 

 to roughly determine the pressure of the steam before open- 

 ing the tap T a small auxiliary mercury gauge was attached 

 to C close to the vessel ; when the pressure as registered 

 by this gauge was constant and had been adjusted to about 

 the value required for taking an observation, the tap T was 

 opened, thus putting the oil manometer in connexion with 

 the vessel. As the tube E was fine very little steam dif- 

 fused into it, and no trouble was experienced from this cause 

 when the experiment was carried out as described. To get 

 rid of any small quantity of moisture which might collect 

 after the apparatus had been working for several hours a 

 T-piece F provided with a drain-tap was attached through 

 which such moisture could be expelled. To carry out an 

 experiment the jacket was filled with steam under pressure, 

 the tap A being open, and the tap D was then opened and 

 steam allowed to enter the vessel, until all the air had 

 been expelled : the taps D and A were then closed. It 

 was found that the pressure in the vessel rose for a short 

 time on account of a small quantity of water carried over by 

 the steam entering through D. The pressure of the steam 

 was then adjusted to a suitable value (about 56 cms. of water 

 above the atmospheric pressure) and allowed to become con- 

 stant. The tap T was then opened, after which the tap A 

 was quickly opened and the pressures and temperatures 

 registered, as in the experiments with air. The initial tem- 

 perature (0i) was always observed just before opening the 

 tap A. only a few seconds being allowed to elapse between 

 taking this observation and opening the tap. Temperatures 

 were read to , 02°C. and pressures to the nearest millimetre of oil. 



