348 DE. J. A. HARKER ON THE HIGH-TEMPERATURE STANDARDS 



IV. Barometer and Auxiliary Measurements. 



The barometer used was a Fortin with f'-inch tube, made by HICKS and 

 standardised at Kew. It was compared from time to time with an old Royal 

 Society's standard and found to have the same correction, '05 millim. 



The temperatures of the mercury columns in the gas thermometer were obtained by 

 means of two auxiliary thermometers disposed at convenient heights, having their 

 bulbs immersed in short test tubes filled with mercury. These are called (2) and (3) 

 in the examples of gas thermometer readings given later. The temperature of that 

 part of the dead space over the mercury in tube A. is given by a thermometer 

 inserted into the metal stopper carrying the reference point. This thermometer is 

 labelled (l). The temperature of the platinum capillary is kept constant by 

 surrounding it with a water-jacket of rubber tube, through which a circulation of 

 water from a supply vessel kept in the room is maintained during the experiments, 

 the temperature of the flow being given by the thermometer (4). 



In making the correction of the pressures of the various mercury columns to 

 latitude 45 and sea level, the latitude of the thermometric laboratory was taken as 

 51 26' 47", and the height above sea level of the cistern of the barometer as 

 10 metres. 



The relation 



G latitude Bushy House , , 



^ . r-"o was taken as 1 '000561. 

 G latitude 45 



V. Preparation of the Gases. 



The nitrogen used in the earliest experiments was obtained from atmospheric air by 

 the ordinary method of absorbing the carbonic acid by potash and the oxygen by hot 

 copper, but for the sake of continuity with the work of CHAPPUIS and the author, in 

 which chemical nitrogen was used in all the high -temperature experiments, the 

 method of preparation employed by them was used in all the later work. Into a 

 flask containing 100 grammes of potassium bichromate, dissolved in air-free distilled 

 water, was introduced gradually by means of a tap-funnel a strong solution containing 

 100 grammes nitrate of soda and 100 grammes nitrate of ammonia. When gently 

 heated on a water-bath this mixture gives off a steady stream of nitrogen, which is 

 passed over hot copper and copper oxide to destroy any oxides of nitrogen it may 

 contain, and collected over weak boiled potash solution in a gas holder. From this it 

 passes through solutions of silver nitrate and strong potash, boiled sulphuric acid, and 

 over solid caustic potash to a larger U-tube containing phosphoric anhydride, where 

 it is left several days before use, or is collected in larger quantity in the dry gas 

 holder over mercury. The tightness of all the joints and taps was ultimately made 

 so perfect that no measurable inward leak of air could be detected by the attached 

 manometer, even when the whole was left standing under reduced pressure for some 

 months. A sketch of the gas preparation apparatus is shown in fig. 3. 



