on the Surface of Water* 533 



A third drop produced active currents in lycopodium powder. 

 A fourth drop added next day was inert until fresh water was 

 added, when it became active, as in the former case. 



The camphor rotations are checked or stopped by any process 

 which diminishes the adhesion of the water \ so also in the case 

 of eugenic acid, its motions are diminished if a soluble salt be 

 added to the water. On a saturated solution of common salt 

 the disk was contorted at the edge and then slowly rotated and 

 moved about, driving before it the lycopodium powder. In 

 about an hour the motion was over, and after 7 hours the disk 

 still remained. The addition of distilled water to the solution 

 did not restore the motion. The addition of 10 drops of 

 sulphuric acid to 2 ounces of distilled water had a similar 

 retarding effect on the motions of the disk of eugenic acid. 



The action of small portions of oil placed on the water has a 

 similar effect on the motions of eugenic acid as on those of 

 camphor. I have already shown* that while a fixed oil perma- 

 nently arrests the motions of camphor, by forming a permanent 

 film on the surface, and so preventing adhesion, a volatile oil, 

 such as turpentine, only arrests the motions during its evapora- 

 tion; or if newly distilled does not arrest them at all, the 

 fragments of camphor skating from the water through the oily 

 film and back into the water indifferently. The rotating disks 

 of eugenic acid behave much in the same manner. When these 

 disks do not prevent the drop of oil from spreading, which is 

 often the case, they repel the film or cut their way through it 

 with the greatest ease ; and when the film is so far spent by 

 solution and evaporation, or changed by the action of the air, 

 the particles that are left on the surface form currents with the 

 eugenic-acid disk, after the manner of the lycopodium powder 

 already described. 



The presence of the various oils on the surface of the water, 

 whether as lenticules or films, has a remarkable influence on the 

 duration of the eugenic-acid disk ; for whereas the eugenic-acid 

 disk on the surface of 2 ounces of distilled water in a glass 

 capsule is about 10 minutest, it may be an hour, or several 

 hours, or even days, if a drop of one of the essential oils be 



* Philosophical Magazine for September 1863, "On the Action of Oils 

 in arresting the Motions of Camphor on Water." 



t The duration was much less than this when I first received the speci- 

 men from Dr. Gladstone. Another specimen of eugenic acid (quite colour- 

 less), prepared by my friend Mr. Hatcher, used the day after it was distilled, 

 was very energetic in its motions, producing currents in the lycopodium 

 particles at a considerable distance from the disk. The duration of the first 

 drop on 2 oz. of distilled water in a glass capsule was 8 minutes, of the 

 second drop 51 minutes, of the third drop 135 minutes. The fourth drop 

 had not disappeared after 43 hours. 



