on the Surface of Water. 413 



ether suspended over water containing* bits of gold leaf repels 

 them by its vapour acting at a distance. He denies that cam- 

 phor on a raft Hoating on water rotates ; while Prevost, on his 

 part, knows nothing of surface-attraction, or of the oil that is 

 said to issue from camphor in contact with water, and which is 

 said to produce rotation by its reaction on the fragment. He 

 has looked in vain for such oil, and believes it exists only in the 

 imagination of the Italian physicist. Carradori replies, " What 

 wonder is it that camphor should cover the water with an oily 

 film, since camphor is itself a very volatile concrete oil V He 

 insists on surface-attraction, and cites this ingenious experi- 

 ment : — A bottle 2 inches in diameter with a neck only 3 lines 

 in diameter was filled with water ; fragments of camphor thrown 

 into the narrow neck did not rotate for want of a sufficient expanse 

 of surface-attraction. Enough water was drawn out by means 

 of a straw so as to lower the surface to the wide part of the bottle, 

 when the camphor rotated briskly on the larger surface. Here, 

 again, the two observers are at variance; for Prevost, in his 

 former paper (14), says that camphor will move in capillary tubes 

 previously cleaned by drawing threads through them, and that 

 lively motions may be seen in them with the aid of a magnifying- 

 glass. 



17. In 1812 we meet with Carradori again 18 . He describes 

 some experiments, based on an observation by Accum, that phos- 

 phorus rotates on the surface of mercury. He gives this as a 

 further illustration of the attraction of surface, the phosphorus 

 covering the mercury with a subtle varnish which gradually 

 arrests the motion; but it may be renewed by filtering the mer- 

 cury. Phosphorus was also found to rotate on the surface of 

 tepid water. 



18. In 1820 Serullas 19 describes the motions of alloys of po- 

 tassium, sodium, &c. on a shallow surface of water 1 or 2 lines 

 deep resting on mercury. Small fragments of the alloy of po- 

 tassium and antimony rotated, disengaging hydrogen, especially 

 from one point : each fragment described a circular path in the 

 opposite direction to the point of greatest liberation of the gas. 

 An alloy of potassium and bismuth rotates on the surface of 

 mercury. An alloy of potassium with lead or tin does the same ; 

 but if water be added the motions are more rapid. The smaller 

 the fragments the more rapid the motions : " on les voit voltiger 

 avec une etonnante vivacite : ou dirait des mouchons retenus 

 dans les pieges, faisant des efforts pour s J en delivrer" 20 . Alloys 



18 Giomale di Fisica fyc. di Brugnatelli, vol. iii. pp. 261, 373 ; vol. iv. 

 p. 29/. 



19 Journal de Physique, vol. xci.p. 172. 



20 Prevost also says of the motions of camphor on mercury, " on eut clit 

 les y voir voltiger," for they scarcely touched the mercury. 



