Theory of the Capillary Electrometer. 335 



Professor Hermann questions the accuracy of my method of 

 analysis when applied to steep curves. 



My answer is that I do not employ it in such cases, preferring to 

 take photographs of sudden changes upon plates moving with suffi- 

 cient rapidity to suitably develope the curves. 



Thus in Professor Burdon Sanderson's paper,* figs. 1, 2, 3, and 4, 

 on Plate 1, and figs. 3, 4, and 5, on Plate 3, were intended to show 

 within the limits of a page the entire course of certain phenomena. 

 I did not analyse them, but simply measured the times of the maxima 

 and minima. The remaining curves, viz., figs. 5 and 6, Plate 1, figs. 

 1 7 on Plate 2, and figs. 1 and 2 on Plate 3, are all suitable for 

 analysis, with the exception of the first phase of fig. 1, which is 

 almost too steep. I have done some thirty or forty of this kind. 



As regards the further criticisms, so far as the physical interpre- 

 tation of the curves is concerned, I can only say that cases did occur 

 in which the maximum E.M.F. of the second (positive) phase 

 exceeded the maximum E.M.F. of the first (negative) phase of the 

 same response. With respect to curves, like those in figs. 3 and 4, 

 Plate II, the part referred to by Professor Burdon Sanderson as 

 the " hump," is not merely the curve of discharge. The actual 

 negatives which I measured show a rise of the meniscus after its 

 rapid downward movement has ceased, and while it is still above the 

 zero line, and a similar rise is plainly visible to the eye after every 

 one of the "spikes " in figs. 1, 2, and 4, Plate 1, which were photo- 

 graphed with the machine moving more slowly. 



It is impossible for the mercury, under these conditions, after 

 approaching the zero line, to recede from, without crossing it, except 

 under the influence of a negative Acting P.D. That is to say, the 

 Impressed E.M.F. must be of the same sign as the charge already 

 in the instrument, but must be of higher potential difference. In 

 some negatives this second rise in followed by a descent more rapid 

 than that of the curve of discharge, and therefore indicating a small 

 positive Acting P.D. I first noticed and called attention to it in 

 connection with the curves illustrating my paperf on the " Time 

 Kelations of the Capillary Electrometer," but refrained from discus- 

 sing its physiological significance. 



* ' Journal of Physiology/ vol. 18, p. 117. 

 f < Phil. Trans.,' A, vol. 183, p. 104* 



