530 Mr. C. Tomlinson on the Motions of Eugenic Acid 



acid, and of the butyrates of baryta, lime, and magnesia, rotate 

 well on water ; and even inert substances smeared with an es- 

 sential oil do the same, — the main condition being the formation 

 and diffusion of a film on the surface of the water in one direc- 

 tion, so as to produce a recoil of the solid in another direction. 



In most of the above cases the film is not visible, except by its 

 effects on the lycopodium powder; but in other cases the film 

 may be made quite evident, as when fragments of paper smeared 

 with an essential oil, or little coracles of paper filled with oil, are 

 set floating on water. The film diffusing from the paper in one 

 direction produces motion in the opposite direction. A bit of 

 solid oil of aniseed produces similar effects, visible to the eye 

 and leaving no doubt as to the truth of the recoil theory in ac- 

 counting for these motions. 



There are certain liquids which also rotate on the surface of 

 water. In general a drop of one of these liquids placed on 

 water has an equal action from the centre outwards in all direc- 

 tions, sufficient to throw the surface of the water into vibration, 

 but the conditions necessary to rotation are absent. Thus a 

 drop of creosote gently delivered to the surface of water from the 

 end of a glass rod, will display a highly agitated cohesion-figure, 

 sailing about over the surface during some minutes*. It often 

 breaks up into separate disks, each of which forms a complete 

 cohesion-figure ; and as these become disposed of by solution, or 

 separate into still smaller disks, the motions become more and 

 more rapid. If the action is not equal all round, the disks begin 

 to rotate with great velocity on a vertical axis until they disap- 

 pear. This is especially the case if a small mote or fragment 

 from the duster be on the surface ; the small disk appears to 

 seize this as a sort of lever, and whirls round with it with great 

 rapidity — the real action, however, being that the mote protects 

 a portion of the edge from diffusing out upon the surface, while 

 the other portions do so with vigour, and in this way the retro- 

 active condition necessary to rotation is established, 



A drop of creosote, of carbolic acid, of oil of cloves, or of 

 pepper, and also of eugenic acid, form cohesion-figures on water 

 consisting in each case of a flattened disk about -f^ths or -j^ths of 

 an inch in diameter, the edge of which is in a peculiar state of 

 rapid vibration, caused by the struggle that is going on between 

 the adhesion of the surface and the cohesion of the disk. The 

 greater or less vigour of this action points out the difference 

 between the figures of these liquids to the eye, while the respec- 



* This is more particularly described in a paper " On the ActioD of 

 Vapours on Films," &c, contained in the Philosophical Magazine for 

 August 1861, and on " Cohesion-Figures," in the Number for October of 

 the same year. 



