on the Surface of Water. 423 



greater rapidity; indeed their gyrations are sometimes so rapid 

 as to make the fragment appear hazy. This also occurs when 

 the ether-sponge is held over the rotating camphor. Flakes of 

 solid acetic acid are amazingly active on water; while the needles 

 of solid carbolic acid have a peculiar rapid jerking kind of 

 motion, not consistent with the reaction of the solution on the 

 fragment. Then, again, the sharply defined character of the 

 perfectly circular disk of ether formed by holding the ether- 

 sponge over the surface of the water (25) seemed to point to 

 the action of a force acting equally around and exterior to the 

 disk. 



39. With respect to the rotations on the surface of mercury 

 in which the camphor &c. are not soluble, the theory is not quite 

 so clear. But I gather from the memoir that the rotations are 

 due to variations in surface-tension consequent on the adhesion 

 of the camphor. This must be very slight ; for Prevost says 

 (note 20 ) the fragments seemed scarcely to touch the surface. 

 I also do not see how Prevost's experiment (14) on the motions 

 of camphor on solid plane surfaces is to be accounted for on 

 this theory. It is very desirable to repeat this experiment; and. 

 I hope some of our microscopists will do so. I also do not see 

 how the case of pure or recently distilled essential oils, occu- 

 pying the surface of the water without interfering with the mo- 

 tions of the camphor (28), is met by the theory, unless it can be 

 said that the oil is bound up, as it were, by its own surface-ten- 

 sion, so as not to interfere with the surface-tension of the water. 

 If this condition be admitted, the fragments are as free to move 

 as if the oil were not present. Although the fragments pass 

 through and cut up the oil, the latter does not lose its lenti- 

 cular form, so that its tension is probably not diminished by 

 the presence of the camphor. 



40. There are a large number of facts contained in, or sug- 

 gested by this memoir (such as those relating to the action of 

 vapours and films on the surface of water), which may perhaps 

 call for a separate notice. But as far as the motions of camphor 

 &c. on the surface of water are concerned, I am bound to admit 

 (notwithstanding 39) that this curious and suggestive problem^ 

 which has occupied so many scientific minds during nearly two 

 centuries, has at length received a satisfactory solution. And 

 this, like every true scientific work, has absorbed a vast number 

 of phenomena which apparently had little or no mutual con- 

 nexion. During these two centuries many labourers have been 

 working in the same field, tilling a difficult soil, which to the 

 most diligent culture never yields a harvest, but only now and 

 then a few grains, for which, it may be, the proper granary is 

 not known, until at length the master comes and collects the 



