ELECTROMOTIVE FORCES IN THE VOLTAIC CELL, 



483 



and unsatisfactory views of physicists in general on the matter (e.g., of 

 Hart) ; but I am bound to say that, so far as I can judge, Dr. von Zahn's 

 own experiments are not of that conclusive and decided character that 

 one had hoped for from his start, and he signally fails to sum up the facts 

 in a neat and crisp manner. He adheres to the contact view, but his 

 adhesion scarcely seems to me to be based on strong evidence ; and in fact 



Fi«. 11. — Von Zalm's Apparatus for experimenting in different Gases at different 

 Pressures and Temperatures, and also in high Vacua. 



The upper plate m is attached by glass roils to a sledge h, which slid rtical steel rods 



<;, being pulled up by a string. On reaching its highest point it comes into contact with 

 an insulated platinum wire s, which communicates with a Hankcl electrometer through 



one of the insulated exits p.. The bottom ]>lafe is supported on the ring e. t is the stop- 

 cock for exhaustion and change of gas. Fig. 2 shows the appendage to the bottom plate 

 for warming it by a current of steam. Tig. 3 shows the apparatus used for high vacua. 

 The two plates are zinc and platinum, and the platinum is arranged to fall by its own 

 weight wdien the whole thing is inverted. The diagram shows it in its highest position, 

 and just going to fall back into its dotted position. The friction of its guide rod seems 

 not wholly satisfactory. 



his theoretical views seem, a little superficial, considering the date at which 

 he writes, so that one may admit pretty well all he says about contact 

 and not chemical action as the cause of the Volta effect without being 

 deeply committed to any specially true or specially false position. 1 His 



t he thermoelectric power of metals the same at one-millionth atmosphere as at 1. 

 This, however, is not conclusive, if it be regarded as a matter needing experiment to 

 settle. 



1 I am afraid this is not peculiar to Dr. von Zahn. It may be that the German 

 writers on the subject are too busy accumulating facts to care much about their pre- 

 cise theoretical bearing, but I notice a very loose and unsatisfactory way of putting, 

 lorward secondary matters, as if they were the real points at issue, and of never real ly 



n2 



