THOMSON'S METHOD OF CHEMICAL ANALYSIS 63 



Helium is associated with a single very strong line of mass 

 4 corresponding to He + . As this gas cannot be removed from 

 the camera by the cooled charcoal the secondary effects are 

 usually very strong. Plate V shows the two faint hydrogen 

 lines and the bright helium line. This plate is an admirable 

 illustration of the danger of secondary rays. The apparent 

 parabola just inside the He parabola, which corresponds to a 

 mass 5 and might easily suggest a compound HeH, is really not a 

 primary at all. If the pressure in the camera be allowed to rise 

 rather higher, the effect shown in Plate VI is produced, bright 

 beams of secondaries of both signs being the only visible rays. 



Mercury.— This element possesses quite peculiar interest 



in connexion with these results. Its presence in the discharge 



tube in small quantities is, of course, to be expected, as the 



apparatus is exhausted by a mercury pump. Should mercury 



not be required, it can be frozen out with liquid air; in general, 



however, its presence is an advantage, as the mercury line 



cannot possibly be mistaken and gives a very valuable standard 



for measurement. The presence of large quantities of certain 



gases, notably oxygen and the halogens, involves its complete 



disappearance. The behaviour of mercury is in two ways 



quite inexplicable : in the first place, although the heaviest of 



all the elements yet measured, its photographic efficiency seems 



almost as great as that of the extremely light elements ; and 



what is still more unaccountable, its parabola invariably seems 



to extend almost to the very origin itself and would require at 



least three or four charges upon a single atom to account for 



its enormous kinetic energy in the manner already indicated. 



Nearly all the Plates here show its characteristic line quite 



distinctly but Plate VII gives the most striking idea of its 



beautiful parabolic form and remarkable appearance when the 



strength of the magnetic field is made extremely high ; the 



electrical displacement due to a single charge can be distinguished 



as a bright " bead " a short distance along it ; the head of the 



other line (CO + ) in the plate is in the same vertical line. This 



mercury line 200 is almost always accompanied by the double 



charged one corresponding to 100, which can be seen plainly 



in Plate IV. Mercury is unique in that it is the only metallic 



element, with the doubtful exception of potassium, which as 



yet has given definite proof of its existence in positive rays. 



Plate VIII, which was obtained from a mixture of hydrogen 



