1878.] 



Contact Theory of Voltaic Action. 



235 



following formulas, which have been employed in obtaining the values 

 of x given above. 

 Without condenser, 



log*=7'5908-log Q logg), 



with condenser, 



log«=4-7216-logglog J), 



t beiug as usual measured in minutes. 



The changes which occur in the conductivity of paraffin wax when 

 it melts are best seen in the curve AB. It is probable that the curve 

 from E to F represents the continuation of AB to a different scale, and 

 we have found that the further continuation beyond the point F is 

 represented by the curve from C to D, which is again drawn to a dif- 

 ferent scale. 



All the loss of charge observations are combined in the curve GCD, 

 and it is quite evident that there is a decided want of continuity in 

 the curve. It is probable that paraffin follows the logarithmic law 

 until it nears the melting point, but after melting the increase of con- 

 ductivity would seem to be proportioned to the increase of tempera- 

 ture. The curve CD may be a straight line, but the galvanometer 

 results give a curve which is decidedly concave upwards ; it is still, 

 however, a very flat curve. 



Solid paraffin during the four degrees before melting alters its 

 specific resistance per cubic centimetre from 34000 X 10 6 megohms to 

 1000 x 10 6 megohms. For the lowest temperature at which it can be 

 said to be melted, its specific resistance seems to be about 10 x 10 6 

 megohms, and 77° '8 C, to be pretty accurately l'354xl0 6 megohms. 



Gutta-Percha. 



The effect of heat on the conductivity of gutta-percha was studied by 

 noting the loss of charge of two copper plates separated by a sheet of 

 gutta-percha 0'191 centim. thick. The conductivity even at ordi- 

 nary temperatures was sufficiently great for the employment of a con- 

 1 F 



denser. The values of - log — -, are shown in the curve OP (fig. 2), 

 t E 2 



the co-ordinates of the points O, a, b, &c, . . . P of the curve are given 

 in the following table, x, the specific resistance per cubic centimetre 

 in megohms has been calculated from the formula : — 



log a=4 ^tf-logg log g). 



The plastic condition of the gutta-percha, at the higher temperatures 

 has indicated itself in the curve between t and P. 



VOL. XXVII. R 



