Thermo-electric Action of Metals in Electrolytes. 283 



One of the objects of the research being to examine the relation of 

 ordinary chemical corrosion to the thermo- and chemico-electric 

 actions of metals in electrolytes, the rates of corrosion of as many as 

 possible of the different metals employed (see Table I, p. 253), wholly 

 immersed in an equal number of separate portions of the same liquid, 

 were ascertained, with one liquid at a time, in order to compare the 

 series thus arrived at with the orders of the same metals in the same 

 liquid in the thermo- and chemico-electric series. 



In carrying out this object, obstacles arose which prevented com- 

 plete corrosion series being obtained to correspond with the thermo- 

 and chemico-electric ones. In some cases the metals became covered 

 with a film of gas, or with a solid coating, each of which altered the rate 

 of corrosion ; these cases were entirely rejected. In other cases the 

 liquids contained traces of impurity which caused a solid film upon 

 the metal ; this was overcome by repeatedly immersing the metal in 

 the liquid until the impurity was wholly removed, and then reim- 

 mersing it several times during longer periods, until concordant rates 

 of corrosion were obtained. And in other instances the metal itself 

 contained small amounts of impurity, which by exciting electric 

 polarity, increased the corrosion. In addition to these and other 

 influences, the rate of corrosion must have been affected by the 

 unequal degrees of adhesion of the soluble products of corrosion to 

 the corroding surface, and the unequal rates of diffusion of those 

 substances into the surrounding liquid ; these influences were reduced 

 to a minimum by employing either very dilute solutions of the 

 corroding agents, or such liquids as corroded but slowly. All these 

 circumstances had to be considered, and each combination of metal 

 and liquid treated in such a manner as to ensure the most attainably 

 accurate result ; but even under the most carefully prepared condi- 

 tions, rate of corrosion is a very variable phenomenon. 



With silver, palladium, gold, and platinum, it was necessary to use 

 much larger sheets and stronger solutions, and to continue the expe- 

 ments during a longer period. The solutions of the two alums were 

 four times, and those of potassic cyanide, sulphuric, hydrochloric, 

 nitric, and chloric acids were of ten times, and the remainder of the 

 usual strength. The silver was immersed in dilute nitric and chloric 

 acids during one month and the other metals during three months: 

 Most of the liquids corroded silver very slightly, and the metal 

 acquired an adherent insoluble coating. The rates of loss are given 

 in the table, but are separated from those in the weak liquids by 

 horizontal lines. 



The action was in no case allowed to continue longer than was 

 sufficient to exhaust about 10 or 20 per cent, of the corroding 

 substance. After excluding all doubtful cases, a sufficient number 

 remained for the purposes intended. The rates of loss by corrosion 



VOL. XXXVIL u 



