98 SCIENCE PROGRESS. 



The sectional area of the water in this trough has been 

 made as nearly as possible 5 cm. 2 , and since each division 

 of the scale is 1 milliampere it follows that each half division 

 indicates a current-density through the water (and through 

 the tadpole) of i«. 



We may now proceed with an experiment. The tad- 

 pole is at rest ; closing the circuit I pass through it a current 

 of 1 milliampere (i.e., of 2a), the animal is agitated and 

 turns its head to anode, since its head is to the anode and 

 its agitation takes the form of swimming movements, it 

 swims to the anode and there stays quiet. I now reverse 

 the current, the animal is agitated, turns round and swims 

 across the trough to the new anode. I reverse again and 

 the tadpole passes over again, always moving oppositely to 

 the pointer, i.e., against the current from kathode to anode. 



This, however, has not been a perfectly typical experi- 

 ment in so far as it has not clearly exhibited what — in 

 agreement with Hermann who first made the observations, 

 and in disagreement with Ewald who contradicted them — 

 I have been led to admit as the fundamental fact. 



It is not always possible to predict the direction in 

 which a given tadpole will swim, nor that which it will 

 finally occupy ; it may and often does swim the other 

 way, i.e., towards the kathode, and there remain more 

 or less quiescent as long as the current is passing. But 

 if following Hermann's plan we observe a number of tad- 

 poles in a large trough (Hermann's trough was 180 x 63 

 mm.) traversed by a moderately weak current (1 to 2a), 

 we shall be able to see that a large number of tadpoles 

 point motionless towards the anode, that others swim to- 

 wards the anode, and that a few point and swim towards 

 the kathode. 



Here is what I find to be a " safe" and convenient form 

 of experiment : two tadpoles are placed in a smaller trough 

 (90 x 30 mm.) in the lantern-field, with a current of about 

 4a, alternated in direction two or three times ; their, for 

 this purpose, somewhat too exuberant vivacity is toned 

 down, so that they no longer dart about upon the smallest 

 provocation. They are somewhat sluggish, their move- 



