72 



PROTOPLASM 



Attention has been drawn above to the peculiar pheno- 

 menon that, on the oil-drops being approached by concentrated 

 sulphuric acid, an extension-current of very short duration 

 appears at first at the opposite pole. Since I have had an 

 idea that this counter-current might be a consequence of the 

 warmth produced on one side by the mixture of the sulphuric 

 acid with water, I tried to clear up this point by some 

 experiments. If a drop of olive oil placed under a thin 

 cover glass in water be approached as close as possible, 

 without, however, touching the cover glass directly, by a 

 brass wire of 1-5 mm. thickness, heated red-hot, such a 

 counter-current towards the heated edge of the drop can be 

 produced very distinctly after some time. The experiment 

 succeeded much better still with a small apparatus which 

 Dr. C. Hilger had the great kindness to construct, con- 

 sisting of a very thin platinum wire with its two halves 

 bent parallel to one another, and heated to the glowing 

 point by the electric cusrent. In this way the applica- 

 tion of warmth could be 

 concentrated at will upon 

 a given spot. On fixing 

 the wire so that its re- 

 curved portion was about 

 1 to 2 mm. away from the 

 drop, and then heating it 

 to a moderate glow, the 

 current described was 

 beautifully shown, and 

 lasted until the electric 

 current was interrupted. 

 The particles of lamp-black mixed with the oil were all 

 carried to the side that was warmed, which was the region 

 of quiescence, and thence they extended in the form of 

 an axial streak through the drop (see Tig. 12). It was 

 observed very distinctly that the rapidity of the superficial 

 current diminished steadily from the heated pole to the 

 opposite one — that is to say, in a manner exactly the reverse 

 of an ordinary extension-current, but in correspondence with 

 what should occur in a reversed extension-current. If then 



Fig. 12. 



