THOMSON'S METHOD OF CHEMICAL ANALYSIS 59 



magnetic deflection being small enough to show the two 

 hydrogen lines. It will be seen that there are five very bright 

 lines in each side of the magnetic zero ; if the most deflected 

 line be of mass unity, taking the squares of their relative 

 deflections, the other lines correspond to masses approximately 

 2, 14, 28, 200. The five lines are evidently due to the 

 hydrogen atom and molecule, to the nitrogen atom and 

 molecule and to the mercury atom respectively, each presumably 

 carrying a single charge. 



All the parabolas end off approximately at the same distance 

 from the vertical axis through the bright undeflected spot : that 

 corresponding to the nitrogen atoms, however, has a distinct 

 " beak " or feebler continuation which ends half as far away and 

 therefore must be caused by particles having twice the kinetic 

 energy of those causing the brighter part. It is quite impossible 

 to suppose that these are due to nitrogen atoms which have 

 fallen through twice the voltage, as the actual maximum voltage 

 of the discharge tube never rose appreciably above that corres- 

 ponding to the tips of the other parabolas ; the most probable 

 explanation is that the atoms of nitrogen forming the extension 

 of the curve carried a double charge + 2e while coming up 

 to the cathode and therefore reached it with twice the normal 

 kinetic energy. If during their passage through the canal- 

 ray tube they picked up a single negative charge — e, they 

 would emerge as atoms with a single positive charge and so would 

 fall upon the same parabola but at a distance half as far away 

 from the magnetic axis. If this view be correct, we might expect 

 some of these doubly charged atoms to retain both charges 

 throughout the fields ; they would then behave exactly as would 

 particles of mass 7 with a single charge + e, as the position 

 of the parabola depends only on the ratio nt/e. On looking for 

 such a parabola, it can be seen clearly between the nitrogen 

 atom and the hydrogen molecule, though it is naturally rather 

 faint. Similar evidence of doubly charged particles will be seen 

 in several of the other plates. 



Though the negative in Plate I is a good one to reproduce 

 in print and to illustrate the general characteristics, it is by 

 no means the best type for actual measurement, as the lines 

 in it are much too thick and bright. It would be quite impossible 

 to reproduce satisfactorily the plates with which the best metrical 

 results have been obtained, as a line can be measured with great 



