19U] on The Coming of Age of the Vacuum Flask 253 



was sealed a small mercuiy manometer of fine capillary tubin<^. The 

 thermometer was filled with pure helium to 273 mm. pressure at 

 O' C. The reading of the mercury manometer thus gives, with 

 slight corrections previously determined, the absolute temperature. 

 It is scarcely necessary to add that, by exhausting the liquid 

 nitrogen, a lower initial temperature than 78° Abs. can be secured. 



The hydrogen evaporating from the liquid in the vacuum 

 vessel C, in which the calorimeter is immersed, is employed in the 

 interval of observations to maintain a hydrogen atmosphere through 

 the neck of the calorimeter and the connected measuring tubes. 

 Risk of solid air in the calorimeter neck is thus obviated. A simple 

 arrangement of a three-way cock T, connected at the top of the 

 brass fitting on the central tube of the cooling vessel, allows this to 

 be manipulated. The hydrogen thus passes continually in at the 

 tubular top of the fitting on the central tube of the cooling vessel, 

 and out through the stopcock on the calorimeter neck, and through 

 the cocks to the measuring vessels. When an observation is to be 

 made, the three-way cock is turned to allow the hydrogen current 

 to escape to the laboratory, thereby closing off the calorimeter, 

 which now only connects to the collecting and measuring vessels 



Y and ^y. 



Y consists of a glass tube 8 cm. in diameter and 40 cm. long, 

 open at the bottom and provided with a wide T-piece at the top. 

 The tube is immersed to the neck in water in a glass cylinder, and is 

 counterpoised by a weight and cord running over a pulley just 

 above. It is thereby readily raised during the time gas is being 

 evaporated from the calorimeter ; this ensures that no back pressure 

 is produced. One arm of the T-piece is open and connects to the 

 stopcock D on the calorimeter neck ; the other is provided with 

 another small stopcock and connects to a 200 c.c. gas burette ^Y 

 similarly immersed in water. Tliis latter stopcock is closed during 

 the collecting of the gas from a dropped piece of material. This 

 being completed, the calorimeter stopcock is closed while the evapo- 

 rated hydrogen gas is measured by transference to the burette, the 

 slight continual evaporation from the calorimeter meanwhile finding 

 a vent through the three-way cock T at the top of the cooling vessel, 

 which is now turned on. These arrangements are necessary to 

 secure the minimum impediment to the evaporating hydrogen, which 

 is usually evolved in less than 10 seconds, any temporary back 

 pressure being fatal to concordant results. At least 15 seconds are 

 allowed for collecting the gas given off, and even longer, in some 

 cases, with badly conducting bodies. 



A slightly different form of calorimeter vacuum vessel was used 

 on some occasions, but without modifying the results. Instead of 

 the constriction at the top of A, a ground conical neck was used of 

 the same diameter as the inner tube. This neck was fitted with 

 a similar cri-ound conical hollow tube sealed on to the tube B. 



