70 PROTOPLASM 



in the resting region ,r, and formed a black axial streak, entirely 

 unlike their behaviour in a drop of pure olive oil. But so 

 complete a collection of the soot at the hinder end, as is peculiar 

 to the paraffin oil, did not occur. 



I would merely point out here briefly that the currents in 

 drops of olive and paraffin oil were also reproduced, as was to be 

 expected, by numerous other fluids in a similar, though less 

 energetic, manner. Since a special study of these processes from 

 a physical standpoint was no part of my task, I only experi- 

 mented on some few fluids for my own information. Absolute 

 alcohol produces extension-currents in both the oils named, which 

 mostly last only a short time, but can usually be called forth 

 again by a repeated addition of alcohol. NaHO or NH 4 produce 

 similar extension-currents in paraffin oil ; in some experiments 

 with paraffin oil and NH 4 performed in the year 1889, the cur- 

 rents lasted above a quarter of an hour. I have recently repeated 

 these experiments with paraffin oil and ammonia, and, as a rule, 

 obtained only feeble currents of short duration, but succeeded 

 finally in reproducing the phenomenon Avith the duration and 

 clearness earlier observed. Concentrated sulphuric acid produces 

 a powerful extension-current in both oils, which can be observed 

 very beautifully in olive oil in particular, and can, as it gradually 

 becomes feebler, be strengthened again several times by fresh 

 addition of sulphuric acid. No visible alteration of the olive oil 

 by means of the sulphuric acid is observable at first; but if 

 the drop be washed out with water its outer zone becomes 

 turbid. Probably, therefore, minute droplets of sulphuric acid 

 have been taken up which do not become distinct until 

 after addition of water. In both oils the appearance of the 

 extension-current caused by the sulphuric acid is preceded in the 

 drop by an exactly opposite current of short duration. This has 

 its centre in the margin of the drop farthest from the sulphuric 

 acid, and the down current passes towards the point of contact 

 with the acid. As has been said, this feeble current only lasts 

 a very short time, and makes its appearance before the sul- 

 phuric acid touches the edge of the drop. I shall try to show 

 below that this current has its cause in the rise of temperature 

 which follows when the concentrated sulphuric acid mixes with 

 the water. 



It seemed to me of interest to try whether the experi- 

 ments described could also be reversed in the following 

 manner. The drop of oil is placed under the cover glass in the 

 fluid with which it shows the smaller surface tension. Then the 

 fluid with which it has the higher tension, namely water, is 

 allowed to flow to it on one side. In this case, according to the 



