arresting the Motions of Camphor on Water. 189 



case of camphor, the motions go on until the whole of the solid 

 is disposed of. If the surface be touched with an oil or any 

 substance that spreads out into a film so as to prevent the cam- 

 phor film from forming, the rotations cease. A little flour from 

 the point of a penknife will spread out over the surface and stop 

 the motions. Oils do this very effectually ; and it was supposed, 

 up to the date of my essay, that any kind of oil would perma- 

 nently arrest the camphor-motions. My experiments led me to 

 the conclusion that volatile oils that leave no residue, arrested 

 the motions only during their solution and evaporation. I now 

 go much further, and show that certain oils do not arrest the 

 camphor-motions at all, and that in such cases the camphor film 

 has a stronger adhesion to the surface of the water than that of 

 the oils in question. 



The reason why the fixed oils stop these motions is, that they 

 form permanent films, displacing the camphor film, and prevent- 

 ing its adhesion to the water. Volatile oils such as turpentine, 

 contain products of oxidation which are neither soluble nor vola- 

 tile ; and these arrange themselves in the form of a delicate per- 

 manent network over the surface of the water, and effectually 

 prevent the adhesion of the camphor fragments. Such products 

 of oxidation can, however, be got rid of by distillation. For 

 this purpose about an ounce of turpentine was distilled from off 

 caustic potash; and when a drop of the distillate was placed on 

 the water among the rotating fragments, the resulting film dis- 

 placed them for a moment, but did not stop the rotation. On 

 the contrary, they immediately invaded the oily-looking film, 

 skating in and through and about it, and leaving long trails free 

 from oil behind. 



This, then, is a case in which the camphor film has a stronger 

 adhesion for the water than the camphene film has. A large 

 number of oils, isomeric with oil of turpentine, and reputed com- 

 mercially pure, were tried, but they all more or less stopped the 

 rotation of the fragments. On rectifying them by distillation, 

 they either did not arrest the motions at all, or those motions set 

 in again as soon as the film had evaporated. 



Oil of bitter almonds is not liable to be acted upon by the air 

 in the same manner as the turpentine oils ; it partly evaporates 

 and partly forms benzoic acid, which is one of the gyrating sub- 

 stances. It is, moreover, displaced by the camphor film ; hence 

 it does not arrest the camphor-motionsc The same remark applies 

 to Persian naphtha, belmontine, Young's paraffine oil, some spe- 

 cimens of rock-oil (rectified) from Canada and Newfoundland. 

 One specimen, of a deep yellow colour, arrested rotation ; it was 

 distilled, and the distillate had no retarding effect. 



In order to stop the camphor-motions, the oily film, whether 



