and of certain Liquids on the Surface of Water. 379 



perties of camphor pointed out by Mr. Lightfoot, and from which 

 he derives his well-written explanation of the cause of these mo- 

 tions when he says, ' the water upon which it floats, being ca- 

 pable of diffusing this vapour more readily in certain directions 

 of the crystalline axis, thereby removes sufficient vapour-pressure 

 at these points for the opposite side to drive about (by recoil) 

 the nicely suspended particle/ " 



10. In my reply to Mr. Trachsel 14 , I state that nearly two 

 years before I had pointed out the more rapid solution of cam- 

 phor on a broken than on a natural surface, and also that the 

 vapour theory was three quarters of a century old. It was first 

 started by Volta in 1787, adopted by Prevost in 1799 under the 

 term jet gazeux, and is now being revived without any experi- 

 mental basis being provided for it. 



11. As to the disturbance of the centre of gravity being the 

 cause of motion in a floating and partly soluble mass, we have 

 only to secure a crystal of some soluble salt to a slice of cork by 

 means of an india-rubber ring, so as to be a little lighter than 

 water; and if this be set floating, the waste will be constantly 

 disturbing the centre of gravity of the mass, so as to produce a 

 rolling over and a slow progressive motion. But no explanation 

 of this kind suffices to explain the gyrations of camphor, benzoic 

 acid, citric acid, &c, which are so rapid under the most favour- 

 able conditions (namely, chemically clean glass and water, and a 

 bright, warm, dry day) as to extinguish the form of the frag- 

 ments and make them appear like a grey cloud on the water. 



12. In 1825 the brothers Weber 15 examined the subject of 

 the camphor motions, and showed that a downy feather smeared 

 with oil at the two ends will rotate on the surface of water. But 

 other observers had already pointed out that an indifferent sub- 

 stance, such as sulphur, glass, earth, sugar, paper, &c. (as indi- 

 cated by Carradori in 1808 16 ), smeared with a fixed oil will rotate 

 on the surface of water. But the first observation of this fact is 

 older still. Franklin, in his celebrated letter to Brownrigg 17 , 

 on the action of oil in stilling the waves, dated November 7, 

 1773, states that while visiting Smeaton near Leeds, he was 

 about to show what he calls " the smoothing experiment " on a 

 pond, when Mr. Jessop, one of Smeaton's pupils, spoke of an 

 odd appearance on that same pond. " He was about to clean a 

 little cup in which he kept oil, and he threw upon the water 



14 Chemical News, September 12, 1863. 



15 Wellenlehre, Leipzig, 1825. 



16 Giornale di Fisica Sfc. vol. i. Pavia, 1808. 



17 Memoirs of the Life and Writings of Benjamin Franklin, edited by 

 his grandson, W. T. Franklin (Lond. 1819), vol. ii. p. 268. Also Phil. 

 Trans. 1774. 



