60 SCIENCE PROGRESS 



accuracy if the canal-ray tube be sufficiently fine even when it 

 is only just visible on the negative. 



For measuring purposes the negative is clamped in a special 

 apparatus and a needle, mounted on a slider so that its point 

 just does not touch the gelatine, is moved across the parabolas 

 in a direction parallel to the magnetic axis O Y (fig. 5); whenever 

 the needle lies exactly over a parabola, its position is read on 

 a vernier scale. In the case of a fine line the position can be 

 determined to about ^ mm. 



In order to give some idea of the measurements which can 

 be made in this way, the actual records of an experiment with 

 a very fine canal-ray tube working satisfactorily may be quoted. 



Gas in discharge tube air at about T ^ mm. pressure. Poten- 

 tial on plates 166 volts. Current through magnet 2*00 amperes. 

 Exposure i£ hours. Discharge potential equivalent to spark- 

 gap 1 § cm. in air; <^is the displacement in mm. from electrical 

 axis ; m is the corresponding mass obtained from the inverse 

 square of d expressed relatively to mercury as 200. 



Probable cause of line. 



Mercury atom with single charge. 



Mercury atom with double charge. 



Very faint line, possibly mercury with triple charge. 



Very faint, probably C0 2 . 



Nitrogen molecule (brightest line). 



Oxygen atom. 



Nitrogen atom. 



Carbon atom. 



Oxygen atom with double charge. 



Nitrogen atom with double charge. 



Carbon atom with double charge. 



Hydrogen molecule. 



(Carbon and its compounds were present as impurities derived 

 from the apparatus; these can only be eliminated with great 

 difficulty by prolonged washing with oxygen. Lines due to 

 such impurities are, as a rule, very faint in comparison with those 

 due to the gases known to be present in quantity.) 



Here we have twelve distinct parabolas, not counting that 

 due to the hydrogen atom which has been thrown off the plate 

 by the large magnetic field. Of these all the bright ones fall 

 exactly on positions expected from the gas that filled the tube, 

 their masses agreeing with the generally accepted molecular 

 and atomic weights to about 1 per cent. 



