404 PROCEEDINGS OF THE AMERICAN ACADEMY. 



than any of those in common use. This method consists in heating mer- 

 curic oxide of high purity obtained in the wet way, and condensing the 

 mercury set free. The oxides of such metals as zinc and cadmium are 

 not decomposed by heat, while metals like platinum and silver will be 

 set free but not volatilized. Mercury is the only readily volatile metal 

 whose oxide is decomposed at red heat. For preparing small quan- 

 tities of very pure mercury this method may, therefore, be recommended. 

 In order to accelerate the decomposition and also to prevent the reunion 

 of mercury and oxygen, the process is best carried on in vacuo. 



After the apparatus was set up and hydrogen had passed through the 

 cell for several days to drive out the enclosed air, measurements were 

 begun, but at first no constant results could be obtained. The current 

 was subject not only to gradual changes but to sudden and capricious 

 fluctuations, which rendered the experiments valueless. This unsatisfac- 

 tory behavior, which may have been due to traces of oxygen clinging 

 obstinately to the cell, or perhaps to incomplete saturation of the platinum 

 electrode with hydrogen, continued for a surprisingly long time, and only 

 after several weeks could definite and constant readings be obtained. 

 Indeed, most of the results noted below were obtained after nearly three 

 months, during which time hydrogen had been passing continuously 

 through the cell at the rate of one cubic centimeter or more per hour. 

 During this time also a current of about five-millionths of an ampere 

 was sent through the cell continuously in order to precipitate any mercury 

 which might have found its way into the solution and especially to help 

 establish constant conditions at the cathode, partly by reducing the last 

 traces of oxygen and partly by the mechanical stirring caused by the 

 bubbles of hydrogen. 



At first it seemed that these bubbles, which, as we have stated, were 

 invariably formed by the current, might cause considerable inconvenience 

 and uncertainty in the measurements. The bubble always started at the 

 line of contact between the glass and mercury, but on becoming large 

 enough to be easily visible to the eye it would mount suddenly to the top 

 of the meniscus and there continue to grow, clinging to the mercury with 

 great tenacity. The only easy way of removing the bubble was to tip 

 the cell so as to bring the meniscus for the moment above the surface of 

 the acid. Fortunately this treatment proved to have no effect whatever 

 on the polarization, so that with a given potential the bubble could, as a 

 rule, be allowed to form and be removed repeatedly without changing 

 the current. Occasionally, however, this was not the case, and therefore, 

 in most series of experiments the current at every potential was read both 

 before and after removing the bubble. 



