on the Surface of Water. 411 



the point where the effluvium is discharged most abundantly, 

 covering the water and suffusing it with colour. It is further 

 shown that when the water becomes impregnated with the cam- 

 phor &c. the motions cease, that warm water and fine weather 

 are favourable to the phenomena, that the purity of the water 

 and of the containing vessel are necessary to success (indeed the 

 success or failure of the experiment is a sort of indication of the 

 purity of the water), that agitation of the water assists the expe- 

 riment, and, lastly, that the gyrations take place on wine but 

 not on spirits of wine, and not very well on olive-oil. 



9. About the year 1794 Carradori 9 began to publish a number 

 of papers and memoirs, suW attrazione di superficie, in which he 

 shows, by a great variety^ of ingenious experiments, that the 

 surface of water exerts a remarkable attractive force on various 

 bodies ; and in 1800, referring to the motions of camphor, he 

 says 10 , " I prove that on this surface-attraction, and on no other 

 cause, the motions of camphor depend." And again, "The me- 

 chanical force of the elastic vapour against the water has nothing 

 to do with the phenomenon ; it depends entirely on surface- 

 attraction ;" and in order to show that a non-volatile body will 

 rotate, he repeats Franklin's experiment (6) on the gyration 

 of bits of paper smeared with a fixed oil and thrown on the sur- 

 face of water. 



10. Several of Carradori's papers are in answer to the theory 

 of B. Prevost 11 , which attributes the motion of camphor and 

 other volatile bodies to the formation of an atmosphere of elastic 

 fluid round them, and to the impact of such fluid on the air. 

 According to Prevost, a fragment of camphor of the size of a pea 

 on a metallic disk four or five lines in diameter, and so placed 

 on water, rotates. 



11. Fourcroy 12 , in reporting Prevost's paper, expressed his own 

 opinion that these motions are due to the attraction of odorous 

 matter both for air and for water, and their solution in one or both. 



12. In 1797 Venturi 13 showed that a column of camphor 

 fixed vertically in water wastes away chiefly at the junction of 

 the air and the water. The oily matter of the camphor covers 

 the surface and evaporates ; and this explains the motion of 

 camphor when free to move. This motion is the mechanical 

 reaction which the oily substance, in spreading on the water, 

 exerts on the camphor itself. 



9 Opus, scelti di Milano, vol. xx. Giornale Fisico di Brugnatelli, vol. 

 vii. &c. 



10 Giornale di Fisica fyc. Pavia, vol. i. p. 97. See also vols, hi., iv., 

 viii., ix., and x. 



11 Annates de Chimie, vol. xxi.p. 254; vol. xxiv. p. 31. 



12 Ibid. 13 Ibid. vol. xxi.p. 262. 



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