442 Prof. J. J. Thomson on the Passage 



mercury we know are molecular compounds — that is, they 

 contain the molecule of mercury and not the atom ; the other 

 alternative would be that the atoms of mercury differ from 

 other atoms in not being charged. 



Further Experiments on the Conductivity of Air. 



Some experiments, in which the arrangement was slightly 

 altered, seem to throw light on the way in which air conducts 

 electricity : the electrodes, instead of being placed as in the 

 previous experiments in a platinum tube closed at one end 

 and heated by a furnace, were placed at opposite ends of a 

 vertical thin platinum tube open at both ends, which could be 

 made white hot by passing the current from a large number 

 of storage-cells through it. With this arrangement it was 

 found that the current through the hot air was very different 

 according as the upper terminal was positive or negative. 

 When the upper electrode was negative there was hardly any 

 deflexion, but when it was positive there was a deflexion of 

 70 or 80 scale-divisions. In this experiment there is no 

 appreciable difference of temperature between the top and 

 bottom of the tube; there is, however, a rush of heated air up 

 the tube ; and the experiment shows that the positive current 

 of electricity travels much more easily against the current of 

 hot air than with it, indicating I think that the current is 

 largely due to convection and that the particles carrying the 

 current travel from the negative to the positive pole. This 

 seems in accordance with the experiments of Lenard and 

 Wolff already mentioned, for they found that negatively 

 charged platinum disintegrated much more readily than un- 

 charged or positively charged platinum. 



When the tube was placed in a bell-jar, filled with one of 

 those gases which conduct electricity well, such as hydro- 

 chloric-acid gas, it made but little difference to the current 

 whether the top or bottom electrode was negative, though on 

 admitting the air this difference was at once detected. This 

 indicates, I think, that the main part of the current through 

 air is carried in a different way from that in which it is 

 carried through one of the better conducting gases. 



Laics of Conduction through Hot Gases. 

 It is known that the conduction through hot air does not 

 obey Ohm's law (Blondlot, Journal de Physique, 2 e serie, t. vi. 

 March 1887). The following experiments, although they 

 cannot lay claim to any very great accuracy, show that the 

 conduction through those gases which conduct electricity 

 well does not depart very much from Ohm's law. The great 

 difficulty in these experiments is to keep the temperature 



