RECENT ADVANCES IN SCIENCE 625 



time taken for the passage of a given quantity of electricity- 

 is longer, so that probably the extraneous combustion which 

 causes the loss additional to that due to the convection of 

 the charge through the arc is greater. In short, doubling the 

 time causes more carbon to burn away than doubling the 

 current. 



But, as mentioned above, it is the work with the short arc 

 which leads to the most interesting result ; no matter what 

 strength of current was used, if the arc was reduced to a very 

 short length, so that the subsidiary burning was of a small 

 order of magnitude, the loss per coulomb from the cathode 

 approached the value 3 x io~ 5 gram (no regularity in the 

 behaviour of the anode in these circumstances was suggested 

 by the curves obtained). The coincidence of this number 

 with the value for the electrochemical equivalent of carbon 

 (3'i09X io*" 5 ), on the assumption that this element is quadri- 

 valent, appears too striking to be accidental and must be closely 

 connected with the mechanism of the arc. 



Various theories of the arc have been put forward in recent 

 years. Fleming, as long ago as 1890, suggested that " the 

 negative carbon is projecting off a torrent of negatively electri- 

 fied carbon molecules and these impinging against the positive 

 carbon wear out a crater in it by a sandblast-like action." 

 Thomson in his Conduction of Electricity through Gases puts for- 

 ward the view that " the cathode is bombarded by positive 

 ions, which maintains its temperature at such a high value that 

 negative electrons come out of the cathode ; these, which 

 carry by far the larger part of the arc discharge, bombard the 

 anode and keep it at incandescence ; they ionise also, either 

 directly by collision or indirectly by heating the anode, the 

 gas or vapour of the metal of which the anode is made producing 

 in this way the supply of positive ions which keep the cathode 

 hot. It will be seen that the essential feature of the discharge 

 is the hot cathode." 



Prof. Duffield discusses these theories in the light of the 

 fresh information elicited by his research. The first inclination 

 is to regard the additional fact referred to above as favouring 

 the earlier view of Fleming, viz. that in the immediate neigh- 

 bourhood of the cathode the entire current is carried by carbon 

 atoms, each of which as it leaves the cathode takes with it 

 four electrons derived from the source of current -supply. The 



