1888.] Development of Electricity by Atmospheric Oxidation. 195 



oxide (or hydroxide) which in the case of zinc and lead further dis- 

 solves in the alkaline liquid forming zincate or plnmbate ; the heat of 

 solution of zinc and lead oxides in caustic soda being unknown, the 

 total heat development cannot be exactly calculated. According to 

 Julius Thomsen, Zn,0 = 85430, and Pb,0 = 50300 gram-degrees, 

 corresponding with the E.M.F's. T837 and 1'081 respectively,* where- 

 fore the E.M.F. due to the chemical action (including formation of 

 zincate and plumbate) must be higher still ; on the other band, the 

 highest values observed in any aeration cell were only 1*645 and 

 0*954 respectively (spongy silver-acetate), whilst values of from 1 to 

 4 deci volts lower still were observed with other plates. Hence the 

 E.M.F. actually generated in these cells falls very considerably short of 

 that corresponding with the chemical change, even under the most 

 favourable circumstances, i.e., when producing only an infinitesimal 

 current ; whilst when producing a somewhat greater current, but still 

 of only small density (not exceeding a fraction of a micro-ampere 

 per square centimetre of aeration plate surface in some cases), 

 running down and marked depreciation of E.M.F. is rapidly brought 

 about. 



Much the same remarks apply to cells set up with sulphuric acid 

 and with ammoniacal fluids ; in the former the nett chemical change 

 is the oxidation of the metal and solution of the oxide in the acid 

 forming the sulphate. Julius Thomsen gives the heat values — 



Zirc Zn,0,S0 3 aq = 106090 gram-degrees = 2 '281 volts- 

 Cadmium. . Cd,0,S0 3 aq = 89500 „ =1*924 „ 

 Copper.... Cu,0, S0 3 aq = 55960 „ = 1 '203 „ 

 Silver Ag 2 ,0,S0 3 aq = 20390 „ = '438 „ 



Whilst the highest observed values in the case of the first three metals 

 fall short of these by 4 to 5 decivolts, and with less active aeration 

 plates the deficiency is much greater. Silver, however, when employed 

 as oxidisable metal, does not show this falling off, but rather the 

 reverse, the highest value observed (platinum black) being about 

 0*58, and the next highest (platinum sponge) about 0'46, both 

 exceeding the E.M.F. calculated from the heat value ; obviously this 

 is due, not to anything connected with the aeration plates, but rather 

 to the large negative value of the thermo-voltaic constantf pertaining 

 to silver in contact with sulphuric acid, evidenced also by the circum- 

 stance observed by us. that when silver is substituted for zinc in a 

 Grove's cell, instead of the E.M.F. being depressed by an amount 



* Taking J = 41*5 x 10*, and the unit C.G.S. current as evolving 0-0001036 

 gram of hydrogen per second, whence the factor for converting gram -degrees into 

 volts is sensibly 4300 x 10~ 8 = '000043 per gram-equivalent. 



f ' Phil. Mag.,' vol. 19, 1885, pp. 1 and 102. 



VOL. XLIV. P 



