426 Sir James Dewar [Jan. %S, 



sufficient security when all is cooled. The diaphragms H then take 

 the temperature of the liquid oxygen bath, absorb all stray radiation, 

 and limit the exposure of the cell to the zenith. 



The baffle dome F (Figs. 6 and 8) provides effective protection 

 against ordinary wind disturbance. It fits air-tight round the rim 

 of the base plate D, and by this means the clean air arising from the 

 slow evaporation from the flask is enclosed as in a protected pocket 

 above the flask neck. The cylindrical shape and curved rim divert 

 direct wind, and the baffle diaphragms (of diminishing aperture) 

 check any eddying disturbance entering from above. Without some 

 such device, in the open air the small pressure fluctuations trans- 

 mitted through the flask neck would cause oscillations of the mano- 

 meter liquid amounting to 1 cm. in four or five seconds on gusty 

 days, while a steady wind would increase the normal " zero " reading 

 of the instrument from about 1 mm. up to 1 cm. or more per minute. 



The protection provided by F is equally necessary to preveut 

 condensation in the flask neck and consequent contamination of the 

 interior by ice or other condensable atmospheric matter. In the 

 five to ten minute periods of equilibration between successive obser- 

 vations, F was covered by a thin metal dome. Within this, on a 

 central light metal stage, was supported a flat lead cup packed with 

 pieces of pumice soaked in concentrated sulphuric acid. The air 

 inside F was thus kept pure and dry, with the result that the flask 

 neck remained quite clean for long periods. During the intervals in 

 which no observations were being taken, the flask neck was closed by 

 the stopper K (Fig. 8) consisting of four or five metallic discs (polished 

 copper or aluminium) equally spaced on a thin central rod or light 

 tube. The uppermost metal disc was large enough to rest on the 

 rim of the flask neck, the others passing easily inside, with a small 

 piece cut out from each to clear the tube E and shutter connecting- 

 rod M, which rested close inside the neck. 



The liquid oxygen in A was kept quite clear by a silk gauze bag 

 containing uranium nitrate, which was hung on the stopper-disc,, 

 and removed all traces of solid impurities by electrical attraction. 

 The following experiment illustrates the efficacy of this device. An 

 unsilvered vacuum vessel half full of filtered liquid air has its clear 

 image projected. When by means of a silvered vacuum siphon 

 dipped below the surface several expirations are bubbled through 

 the liquid, the transparency is very rapidly replaced by a thick brown 

 fog. A bag of uranium nitrate is then introduced and moved 

 about in the liquid ; the brown fog rapidly clears up, and in about 

 a minute the clear transparent image is reproduced along with the 

 black projected mass of the coated bag. When the bag is lifted out 

 and held in the lantern beam it is seen to be thickly coated with 

 the white solid impurities removed from the liquid air. 



With the apparatus filled and settled and closed as described, the 

 shutter could be raised without producing any effect, thus proving 



