TRANSACTIONS OF SECTION A. 535 



and in the negative, like K, &c., a diminution of conductivity. Each class 

 again falls into two sub-classes — (a) sensitive substances which undergo self^ 

 recovery, and (6) sensitive substances which do not. In the case of self-recovering 

 substances the conductivity distortion varies with the intensity of radiation. 

 Under the continued action of radiation the distortion attains a maximum, 

 balanced by force of restitution, and on the cessation of radiation there is an 

 elastic self-recovery. 



(2) The three classes of substances, positive, negative, and neutral, may be 

 distinguished by their peculiar characteristic curves. 



(3) The change produced in the sensitive substance by the action of radiation 

 is not, normally speaking, chemical. 



(4) The conductivity change is produced, not only by very rapid, but also by 

 comparatively slow electric variation. Generally speaking, aU the conductivity 

 variation effects produced by electric radiation can be reproduced by compara- 

 tively slow cyclic electro-motive variation. 



(5) Electric conduction in metallic particles sensitive to electric radiation does 

 not obey Ohm's law. The conductivity is not constant and independent of the 

 electro-motive force, but varies with it. In the positive class the characteristic 

 curve — in which the ordinates represent the currents, and the abscisste the 

 electro-motive force — is concave to the axis of the current. The conductivity 

 increases continuousli/ with increasing electro-motive force. The variation of 

 conductivity in the lower portion of the curve is small, but increases with great 

 rapidity in the upper portion. 



(6) The curve obtained with strong is steeper than that with feeble initial 

 current. 



(7) There is found, especially when the initial current is feeble, a critical 

 electro-motive force, at which the conductivity change becomes so rapid as to 

 produce an almost abrupt bend in the curve. Stronger initial current appears, 

 not only to lower the critical point, but also to mitigate the abruptness of this 

 change. 



(8) The effect of electro-motive force in modifying the conductivity of the 

 conducting layer is well seen in self-recovering substances. There is a definite 

 conductivity corresponding to a definite electro-motive force. As the electro- 

 motive force is increased, the sensitive molecular layer is strained, and a definite 

 increase of conductivity produced. When the increased stress is removed the 

 corresponding strain also disappears, and there is an elastic recovery of its former 

 molecular and conductive state. Hence when it is carried through a complete 

 cycle of electro-motive variation, with moderate speed, the forward and return 

 curves coincide, and the substance remains, at the end of the cycle, in its original 

 molecular condition. 



(9) This is the case where there is complete recovery on the removal of the 

 stress. With non-recovering substances we find an outstanding residual effect. 

 In a curve taken with cyclic electro-motive variation the forward and return 

 curves do not coincide, but enclose an area. There is a hysteresis. The larger 

 the range of the electro-motive variation, the greater is the area enclosed. 

 There is a residual conductivity variation at the end of the cycle which may be 

 dissipated by vibration. 



FRIDAY, SEPTEMBER 1.3. 



Department II. — Astronomy. 

 Chaikman: Professor H. H. Tuenek, D.Sc, F.E.S. 



The Chairman delivered the following Address : — 



It was hoped, as you are doubtless all aware, that this Chair would be taken 

 by the Astronomer Royal for Scotland, Dr. Copeland; but unfortunately illness 

 has prevented him com"ing to the Meeting. In doing what I can to fill his place 



N n2 



