Jan. 24, 1884] 



NATURE 



301 



sideiably increased the potential of metals thermo-electro- 

 negative in liquids, and somewhat incl•ea^ed that of those potitive 

 in liquids. 



The electric potential of metaU', thermo-electro-positive in 

 weak liquids, was usually about 3 87 times, and in strong ones 

 I '87 times, as great as of those which were negative. The 

 potential of the strongest thermo-electric couple, viz. that of 

 aluminium in weak solution of sodic phosphate, was '66 volt for 

 100' F. difference of temperature, or about 100 times that of a 

 bismuth and antimony couple. 



Heating one of the metals, either the positive or negative, of 

 a voltaic couple, usually increased their electric difference, 

 making most metal more positive, and some more negative; 

 whilst heatinJ the second one also, usually neutralised to a large 

 extent the effect of heating the first one. The electrical effect 

 of heating a voltaic couple is nearly wholly composed of the 

 united effects of heating each of the two metals separately, but 

 is not however exactly the same, because whilst in the former 

 case the metals are dissimilar, and are heated to the same 

 temperature, in the latter they are similar, but healed 

 to different temperatures. Also, when hean'ng a voltaic pair, 

 the heat applied to two metals, both of which are previously 

 electro-polar by contact with each other as well as by contact 

 with the liquid ; but when heating one junction of a metal and 

 liquid couple, the metal has not been previously rendered electro- 

 polar by contact with a different one, and is therefore in a some- 

 what different state. When a V' iltaic combination, in which the 

 positive metal is thermo negative, and the negative one is thermo- 

 positive, is heated, the electric potential of the couple diminishes, 

 notwithstanding that the internal resistance is decreased. 



Magnesium in particular, also zinc and cadmium, were greatly 

 depressed in electromotive force in electrolytes by elevation of 

 temperature. Reversals of position of two metals of a voltaic 

 couple in the tension series by rise of temperature were chiefly 

 due to one of the two metals increasing; in electromotive force 

 faster than the ether, and in many cases to one metal increasing 

 and the other decreasing in electromotive force, but only in a 

 few cases was it a result of simultaneous but unequal diminution 

 of potential of the two metals. With eighteen different voltaic 

 couples, by rise of temperature from 60° to 160° F. , the electro- 

 motive force in twelve cases was increased, and in six decreased, 

 and the average proportions of increase for the eighteen instances 

 was 'lo volt for the 100° F. of elevation. 



A great difference in chemical composition of the liquid was 

 attended ly a considerable change in the order of the volta- 

 tension series, and the differences of such order in two similar 

 liquids, such as solutions of hydric chloride and potassic chloride, 

 were much greater than those produced in either of those liquids 

 by a difference of 100° F. of temperature. Difference of strength 

 of solution, like difference of composition or of temperature, 

 altered the order of such series with nearly every liquid ; and 

 the amount of such alteration by an increase of four or five 

 times in the strength of the liquid was rather less than that 

 caused by a difference of 100° F. of temperature. Whilst also 

 a v.ariation of strength of liquid caused only a moderate amount 

 of change of order in the volta tension series, it produced more 

 than three times that amount of change in the thermo-electric 

 tension series. The usnal effect of increasing the strength of 

 the liquid upon the volta-electromotive force was to considerably 

 increase it, but its effect upon the thermo-electromotive force 

 was to largely decrease it. Thedegiee of potential of a metal 

 and liquid thermo-couple was not always exactly the same at the 

 same temperature during a rise as during a fall of temperature ; 

 this is analogous to the variations of melting and solidifying points 

 of bodies under such conditions, and also to that of supersatura- 

 tion of a liquid by a salt, and is probably due to some hindrance 

 to change of molecular movement. 



The rate of ordinary chemical corrosion of each metal varied 

 in every different liquid ; in each solution also it differed with 

 every different metal. The most chemically positive metals 

 were usually the most quickly corroded, and the corrosion of 

 each metal was usually the fastest with the most acid solutions. 

 The rate of corrosion at any given temperature was dependent 

 both upon the nature of the metal and upon that of the liquid, 

 and was limited by the most feebly active of the two, usually 

 the electrolyte. The order of rate of corrosion of metals also 

 differed in every different liquid. The more dissimilar the 

 chemical characters of two liquids the more diverse usually was 

 the order of rapidity of corrosion of a series of metals in them. 

 The order of rate of simple corroMon in .any of the liquids 



examined differed from that of chemico-eleclric and still more 

 from that of thermo electric tension. Corrosion is not the cause 

 of thermo-electric action of metals in liquids. 



Out of fifty -eight cases of rise of temperature the rate of 

 ordinary corrosion was increased in every instance except one, 

 and that was only a feeble exception — the increase of corrosion 

 from 60° to 160° F. with different metals was extremely variable, 

 and was from 15 to 321 '6 times. Whether a metal increased or 

 decreased in thermo-electromotive forte by being heated, it in- 

 creased in rapidity of corrosion. The proportions in which 

 the mo^t corroded metal was also the most thermo-electro- 

 positive one was 65'S7 per cent, in liquids at 60° F. and 69' 12 

 in the same liquids at 160° F. ; and the proportion in which it 

 was the most chemico-electro-positive at 60" F. was 84 '44 per 

 cent., and at 160" F. 8077 per cent. The proportion of cases 

 therefore in which the most chemico-electro-negative metal was 

 the most corroded one increased from IS'5'J '0 I9'23 percent, 

 by a rise of temperature of 100° F. Comparison of these pro- 

 portions shows that corrosion usually influenced in agreater degree 

 cheniico-electric rather than thermo-electric actions of metals in 

 liquids. Not only was the relative number of cases in which 

 the volta-negative metal was the most corroded increased by rise 

 of temperature, but also the average relative loss by corrosion 

 of the negative to that of the positive one was increased from 

 3'ii to 6'32. 



The explanation most consistent with all the various results 

 and conclusions is a kinetic one : — That'metals and electrolytes are 

 through' lut their masses in a state of molecular vibration. That 

 the molecules of those substances, being frictionless bodies in a 

 frictionless medium, and their motion not being dissipated by con- 

 duction or radiation, continue incessantly in motion until some cause 

 arises to prevent them. That e.ach metal (or electrolyte), when 

 unequally heated, has to a certain extent an unlike cla^s of mo- 

 tions in its differently heated parts, and behaves in those parts 

 somewhat like two metals (or electrolytes), and those unlike 

 motions aie enabled, through the intermediate conducting por- 

 tion of the substance, to render those parts electro-polar. That 

 every different met.al and electrolyte has a different class of 

 motions, and in consequence of this they also, by contact alone 

 with each other at the same temperature, become electro-polar. 

 The molecular motion of each dilterent substance also increases 

 at a different rate by rise of temperature. 



This theory is equally in agreement with the chemico-electric 

 results. In accordance with it, when in the case of a metal and 

 an electrolyte, the tw o classes of motions are sufficiently unlike, 

 chemical corrosion of the metal by the liquid takes place, and 

 the voltaic current, originated by inherent molecular motion 

 under the condition of contact, is maintained by the portions of 

 motion lost by the metal and liquid during the act of uniting 

 together. Corrosion therefore is an effect of molecular motion, 

 and is one of the modes by which that motion is converted into 

 and produces electric current. 



In accordance with this theory, if we take a thermo-electric 

 pair consisting of a non-corrodible metal and an electrolyte (the 

 two being already electro-polar by mutual contact), and heat one 

 of their points of contact, the molecular motions of the heated 

 end of each substance at the junction are altered ; and as 

 thermoelectric energy in such combinations usually increases by 

 rise of temperature, the metal and liquid, each singly, usually 

 becomes more electro-polar. In such a case the unequally heattd 

 metal behaves to some extent like two metals, and the unequally 

 heated liquid like two liquids, and so the thermo-eleciiic pair 

 is like a feeble chemico-electric one of two metals in two liquids, 

 but without corrosion of either metal. If the metal and liquid 

 are each, when alone, thermo-electro-positive, and if, when in 

 contact, the metal increases in positive condition faster than the 

 liquid by being heated, the latter appears thermo-electro- 

 negative, but if less rapidly than the liquid, the metal appears 

 thermo-electro-negative. 



As also the proportion of cases is small in which metals that 

 are positive in the ordinary thermo-electric series of metals only 

 become negative in the metal and liquid ones (viz. only 73 out 

 of 286 in weak solutions, and 48 out of the same number in 

 strong ones), we may conclude that the metals, more frequently 

 than the liquids, have the greatest thermo-electric influence, and 

 also that the relative largeness of the number of instances of 

 thermo-electro-positive metals in the series of metals and liquids, 

 as in the series of metals only, is partly a consequence of the 

 circumstance that rise of temperature usually makes substance 

 — metals in particular — electro-positive. These statements are 



