342 



Prof. W. M. Thornton. 



[Jan. 12, 



in origin. It will, however, be seen that the form of the curve follows 

 closely a rectangular hyperbola with a vertical axis displaced to about 

 O05 ampere per square centimetre. There is, according to this, a current 

 density at which no sterilisation occurs, however prolonged the exposure, 

 which may explain Zeit's results, the warming action of the current 

 encouraging growth. On the other hand, the fact that the curve is hyper- 

 bolic in contour would suggest that the result depends in some measure upon 

 the quantity of electricity passed through the liquid, since this is the same 

 for each point on the curve, if reckoned from the displaced vertical axis. It 

 is probable that the ultimate cause of the results recorded here is not simple, 

 but is in part thermal, and in part direct electrical action upon the molecular 

 structure of the organism. That it is largely thermal is shown by the fact 

 that longer time is required for sterilisation when the temperature is kept 

 at a lower maximum. On the other hand, if it were entirely thermal, the 

 time should be inversely proportional to the square of the current, which is 

 not the case under the conditions of the experiment. 



7. The Action of Ultra-violet Light on Bacteria. 

 The results of Sections 4, 5, 6, show that there is not a wide difference 

 between the bactericidal influence of direct and alternating currents when 

 the frequency of the latter is low. When, however, bacteria are exposed to 

 electrical oscillations at their highest possible frequency, namely, that of 

 ultra-violet light, the power required for sterilisation is so small that the 

 result can only be caused by selective absorption of energy, either by the 

 organism as a whole, or by its molecular structure. Exposure of B. typhosus 

 in a thin hanging drop of water to strong polarised ultra-violet light causes 

 no marked orientation, which might be expected to occur if the resonance 

 were with the cell as a whole, for the free period of electrical oscillation on a 

 rod is not the same longitudinally and transversely, and the bacillus would 

 be forced, as it is in lower frequency currents, into such a position that the 

 energy absorbed would be greatest. Under favourable conditions, with 

 bacteria fresh from the animal, feeble though clear orientative response to 

 the stopping and starting of polarised ultra-violet light has been obtained, 

 but from the jerky type of movement, distinct from Brownian movement, it 

 would appear to have been derived from stimulus of the flagella rather than 

 to be an orientation by electromechanical forces. The bactericidal maximum 

 found by Marshall Ward was at a wave-length of 4300 tenth metres, corre- 

 sponding to a frequency of 7 x 10 14 , in the low violet. If another maximum 

 should ever be found near a wave-length of 2730, it would clearly point to 

 the action being molecular. The frequency of rotation of an electron doublet 



