Impurity found in Nitrogen G-a* derived from Urea. 99 



as a sort of Geissler pump. The two halves of the apparatus being 

 independent and similar, it will suffice to speak of that which contained 

 the gas to be investigated. The tubes in which the levels of mercury 

 are observed are about 1 cm. in diameter. The fixed one, correspond- 

 ing to the " pump-head " of a Geissler or Topler, is 33 cm. in length, 

 and is surmounted by a three-way tap, allowing it to be placed in com- 

 munication either with the optical tube or with one of narrow bore 

 ending in a U, drowned in a deep mercury trough. The bottom of 

 the fixed tube, prolonged by 92 cm. of narrower bore, is connected 

 through a hose of black rubber with the movable manometer tube. 

 The latter is 70 cm. long and of one bore (1 cm.) throughout. It can 

 either be held in the hand or placed in a groove (parallel to the fixed 

 tube) along which it can slide. The four columns of mercury stand 

 side by side, and the levels are referred by a cathetometer to a metre 

 scale which occupies the central position. It is not proposed to 

 describe the cathetometer in detail, but it may be mentioned that it is 

 of home construction, and is mounted on centres attached to the floor 

 and ceiling of the room. It sufficed to record the levels to tenths of 

 millimetres. The whole apparatus was constructed by Mr. Gordon. 



If the glasses closing the optical tubes were perfect, there would be 

 coincidence of bands corresponding to complete exhaustion of both 

 optical tubes. A correction could be made for the residual error once 

 for all determined, but it is safer to make two independent settings, 

 one at pressures as nearly atmospheric as the case admits, and a second 

 at minimum pressures. There are then in all eight readings to be 

 combined. An example may be taken from a case already referred 

 to: 



I. II. III. IV. 

 9770 9371 9749 9790 

 7272 2165 2469 7445 



Columns I, II refer to the anomalous nitrogen, III and IV to the 

 dried air used as a standard of comparison. I and IV are the fixed 

 manometer tubes in communication with the optical tubes. The re- 

 duction may be effected by subtraction of the rows : 



2498 7206 7280 2345 



Thus 4708, the difference between II and I, of the nitrogen balances 

 4935, the difference between III and IV, of air. The refractivity 

 referred to air is accordingly |^nrl> or 1*048. 



In this example the range of pressures for the air is 493'5 mm., or 

 about two-thirds of an atmosphere. 



Great care is sometimes required to ensure matching the same bands 

 in the two settings. A mistake of one band in the above example would 

 entail nearly 2 per cent, error in the final result, inasmuch as the whole 



VOL. LXIV. I 



