252 Sir James Dewar [Jan. 16, 



testing the behaviour of the cell under different circumstances at 

 different temperatures, as was also carbonic acid. 



While the cell was being saturated, or settling between readings, 

 stopcock («) was set as shown — i.e. open to the gas reservoir at 

 constant pressure, or open to the atmosphere ; (b) was also in the 

 same position as (a) open to the air (generally through a small drying 

 bulb of fresh soda lime). When a reading was to be taken on E, (c) 

 was closed, and (a) turned to connect the cell to the manometer 

 only. A blank reading was then made, with the cell shutter closed, 

 to test fcr equilibrium in the cell. The " zero " of the instrument, 

 thus taken, did not exceed 2 or 3 mm. displacement, and usually was 

 of the order of 1 mm. or less : (a) and (c) were then once more 

 opened to equilibrate everything. In order to make an observation 

 the cock (c) was closed, and the shutter raised before turning (a) to 

 connect with the manometer E. (This was to avoid the fluctuations 

 of pressure caused by eddies in the liquid due to the moving shutter 

 and transmitted through the elastic membrane.) The stopcock (a) 

 was then turned as before, and at the same moment the stopwatch 

 was started. At the end of J minute (or whatever the period 

 required) (a) was reversed to open the cell to H, etc., and close the 

 connection to the manometer E. The shutter was then lowered and 

 the cell left for about 10 minutes to equilibrate. The manometer 

 liquid remained in its displaced position until (c) was again opened, 

 thus allowing ample time for the reading to be taken. In the same 

 way for readings on the horizontal manometer G, (b) was turned to 

 the position shown, with (a) set as first described ; (c) remained open, 

 and the reading was taken as before by turning (a) at the same time 

 as the stopwatch was started, after raising the shutter. 



In Fig. 5 are shown three alternative arrangements of cell and 

 bath in more detail than is possible in Fig. 4. They consist essen- 

 tially of inner and outer vessels (charged with liquid air or nitrogen, 

 oxygen, etc.), the only purpose of the outer one being to arrest 

 heat influx to the inner one, and thus to reduce to a minimum any 

 fluctuations of the temperature of the cell immersed therein. The 

 inner vessel of A, Fig. 5, with a straight central tubular outlet 

 below was constructed of quartz, for the purpose of supporting the 

 thin metal tube connecting the cell to the manometer : this arrange- 

 ment left the whole space above the membrane free from obstruction. 

 The vacuum isolation of this vessel did not extend up beyond two- 

 thirds of its height, thus allowing the outer liquid air to maintain 

 complete contact, and ensuring a uniform temperature throughout 

 the inner vessel. The upper part should preferably be conical in 

 shape, in order to cut off the radiation from the uncooled parts which 

 otherwise would affect the cell when the shutter was raised. The 

 simple shutter shown was lifted vertically for an exposure (the outer 

 portions of the cell being covered with a ring diaphragm) ; but a 

 properly hinged shutter was also used, fixed to a light sleeve fitting 



