THEORY OF TENSION OF THE VOLTAIC BATTERY. 35 



finity of the zinc for oxygen only, but by all the affinities that can take place among all 

 the bodies in the exciting cell. We are, therefore, here led to expand Dr. Faraday's 

 theory, and to regard what follows as directly opposed to the theory of contact. 



305. Upon these principles, in an ordinary arrangement of copper, zinc, and sulphuric 

 acid, the tension of the current is determined, by the sum of the affinities of zinc for 

 oxygen, and hydrogen for copper, diminished by the sum of the affinities of copper for 

 oxygen, oxygen for hydrogen, and hydrogen for zinc. 



306. But as, under all ordinary circumstances, the affinities of hydrogen for zinc and 

 copper may be neglected, they being exceedingly small in comparison with the others, 

 we may assume, 



307. That the tension of the current is equal to the affinity of oxygen for zinc, di- 

 minished by the sum of the affinities of hydrogen and copper respectively for oxygen. 



308. If now we introduce into the exciting cells nitric acid or sulphate of copper, the 

 affinity of the nascent hydrogen for oxygen is satisfied, and the resistance from this 

 source is nearly exterminated, and the tension of the current is then equal to the dif- 

 ference of the affinities of zinc and copper for oxygen. 



309. By thus exterminating the resistance arising from the affinity of hydrogen for 

 oxygen, we succeed in raising the tension greatly ; if next we get rid of the affinity of 

 copper for oxygen, the tension ought to become still higher. This may, in a measure, 

 be effected by making use of a plate of platina, as I found experimentally. 



310. In all these cases, in which the tension increases without loss of quantity, we 

 directly trace the action to a disturbance in the exciting cells. In ordinary voltaic ar- 

 rangements, the maximum tension is never reached, because the affinity of zinc for ox- 

 ygen, which determines the current, is counteracted to a certain extent by the affinity 

 of oxygen for hydrogen. If we satisfy that affinity, an increase of tension is the result, 

 and accordingly as this is more and more nearly effected, more and more of the hydro- 

 gen that ought to be evolved disappears. This remarkable disappearance of hydrogen 

 has been heretofore noticed, but the true office it served has not been detected. If a 

 battery is charged with nitrosulphuric acid, the hydrogen evolved is no longer the equiv- 

 alent of the zinc expended ; in point of fact, the gas evolved is no longer hydrogen, but 

 a mixture of hydrogen and the binoxide of nitrogen, as is proved by its burning with a 

 green flame. I took a small pair of plates, the zinc being amalgamated and the platina 

 freshly cleaned, and placed them in a mixture of six ounces of water and one drachm 

 of sulphuric acid, arranging an inverted tube over them, so as to collect the gas from 

 the platina plate. I determined by weighing the zinc plate how much was expended 

 in evolving a given quantity of gas; and then successively adding sulphuric acid until 

 the total amount had reached eight drachms, it appeared that in each instance it re- 

 quired very nearly T79 grains of metal ; but on adding one drachm of nitric acid to 

 the mixture, the quantity expended rose at once to 2'25 grains, and on adding a 

 second, to 3'00 grains. 



311. Therefore, unless care is taken that no oxidizing body is present, the voltame- 

 ter will give deceptive results. This important precept should be perpetually borne 

 in mind by those who employ it in investigations. A few drops of nitric acid will at 



