1878.] 



Contact Theory of Voltaic Action. 



227 



peratures. Cold glass is acted on by hydrofluoric acid, and this is 

 possible on account of the great electromotive force and on account of 

 the silicon tetrafluoride formed by the combination being volatile, 

 and, therefore, possessing great mobility. Hitherto chemists have 

 only employed the two ideas of chemical affinity and the amount of 

 chemical action, but we have shown that these ideas are simplified 

 when regarded as electromotive force of contact and currents of 

 electricity. To connect the two ideas we have a third, viz., resistance, 

 and the electrical law of Ohm becomes the chemical law — the quantity 

 of chemical action in unit time equals the sum of a great number of 

 terms, each of ivhich is an electromotive force divided by a resistance. 



Gutta-Percha. 



The metallic plates of the last experiment were carefully cleaned, 

 and the paraffin wax replaced by a sheet of gutta-percha, previously 

 scraped, washed with soap and water, ordinary water, alcohol, sodium 

 carbonate, and hot distilled water, and thoroughly dried. The upper 

 plate A was of clean zinc as before. Our experiments on the con- 

 ductivity of gutta-percha, described further on, show that it increases 

 with the temperature, not only at the low temperature at which it has 

 hitherto been examined, but also after it has become plastic at higher 

 temperatures. 



At the lowest temperature at which we examined the arrangement 

 there was a decided charging of the plates, but it was so slow that we 

 must regard the measurement of the minimum electromotive force 

 023 volt as being too small, for a careful examination of the defects 

 of the electrometer showed that under these circumstances the 

 measurement might be too small. At higher temperatures the maxi- 

 mum charge was established with much greater rapidity. 



On each day, some time after the commencement of the experiment 

 for that day, the electromotive force having remained constant for 

 some time suddenly increased, and then remained nearly constant at 

 its higher value. This seemed at first to be due to the plates having 

 been short-circuited for intervals of four minutes or more, but we 

 afterwards found that it occurred when the plates were only short- 

 circuited for a few seconds to get to zero. Also, it did not seem to 

 occur at a particular temperature ; on one occasion the sudden rise 

 happened at about 35° C, and on another at about 72° C. On the 

 first of these days the mean low electromotive force was 0'584 volt, 

 and the mean high 0'763 volt, the very lowest and very highest being 

 054 and 086 volt. On the second day the mean low was 0534 

 volt and the mean high 0*773 volt, the lowest and highest being 

 0*529 and 0*789 volt. We are sure, when these measurements were 

 made, that the conductivity of the gutta-percha was sufficiently great 

 to eliminate any effects due to the minute defects of the electrometer. 



