of Substances upon Voltaic Action. 455 



current in the first case and decrease it in the second, thus 

 proving that the influencing bars acted upon the electro- 

 motive force and not upon the conduction-resistance ; had the 

 whole or even the greater portion of the effect been to diminish 

 the amount of conduction-resistance, the current would have 

 been increased in each case. This experiment of placing the 

 copper bars near the negative electrodes was repeated by 

 another 3b* observations on a subsequent day, and the same 

 result — viz. diminution of the electro-negative state — was 

 obtained. 



Degrees of Influence of Various Substances. 

 The apparatus having been made sufficiently perfect, and 

 the mode of treating the observations satisfactory, numerous 

 series of experiments were made to approximately ascertain 

 the relative magnitudes of effect of equal volumes of different 

 substances, by employing pairs of plain rectangular bars, each 

 bar being 22 inches long, If inch wide, and 2 inches deep, 

 placed above each other at a distance of -f$ inch apart, 

 so that the glass tubes might slide without friction into 

 and out of the horizontal space between them. The sub- 

 stances used included copper, lead, bismuth, antimony, cast 

 iron, wrought iron, brass, magnesium, zinc, flint glass, slate, 

 ebony, deal, and gypsum-plaster — every one of which gave 

 the usual kind of effect. 



It usually occupied five days to obtain the final average 

 value of effect of a substance. In order to make proper 

 correction for the value of the voltaic current alone, which 

 was usually the largest element (unless the solution had been 

 recently made) , an average value of that current alone was 

 obtained on the first day ; a ditto for that of the current plus 

 the effect of the substance on the second and third days ; for 

 the voltaic current alone on the fourth day ; and for the 

 current plus influencing substance on the fifth day. 



On the first day, the tubes being 5 inches distant from the 

 bars and the circuit having been left open all night, the latter 

 was closed in the morning, and after five minutes the spot of 

 light, having become steady, was brought to zero on the scale. 

 Observations were then taken every fifteen minutes, both of 

 the temperature of the tubes and the amount of deflexions. 

 Immediately after each observation the circuit was opened 

 during a few seconds, the needles allowed to become steady, 

 the deflexion again noted, the first deflexion corrected for 

 the amount of the second one, and the true amount thus 

 obtained. This was done about forty times, and the average 

 value of the corrected current obtained by taking the 

 average pf all the observations. 



