Action of Oils in arresting the Motions of Camphor on Water. 1 87 



of moderate violence occurred during the hail- storm, each of which 

 was attended by a redoubled fall of hailstones" (ibid. vol. iv.). 

 Tessier, in his c Notes on the Locality in France ravaged by the 

 Hail-storm of 1788/ remarks, "the hail followed immediately 

 after the lightning and thunder" {Memoir es de V 'Academie,1789). 

 We might cite many other facts in illustration ; some others are 

 quoted in the memoir from which this extract is derived, but 

 those already referred to suffice to show that hail is in all proba- 

 bility formed at the moment of the electric discharge from the 

 storm-cloud. 



This theory does not require the supposition of the presence 

 of two clouds, which frequently does not happen. It is not ne- 

 cessary to suppose the existence of two contrary winds, which 

 often is not met with. Again, it is not necessary to suppose the 

 storm-clouds to be at a very great elevation. On the contrary, 

 this hypothesis furnishes an explanation why hail falls in these 

 climates during the summer and at the hottest portions of the 

 day ; it is because at such periods, when the air is most dry, the 

 electric tension becomes more considerable than in strata of air 

 of a higher elevation, and at hours of the day when the tempe- 

 rature is less. 



In fine, the former of the two portions of this theory is not 

 hypothetical but a demonstrable fact ; whilst, as to its latter por- 

 tion, it may be remarked that if it be indisputable that hailstones 

 are almost instantaneous in their formation, their cause also 

 must be equally instantaneous, and there is no other cause than 

 the one suggested which can operate iu such a manner. 



XXVI. On the Action of Oils in arresting the Motions of Camphor 

 on Water. By Charles Tomlinson, Lecturer on Physical 

 Science ^King's College School, London*. 



AMONG the numerous physicists who have studied the 

 motions of camphor on water, from the time of Volta in 

 1787 to that of Dutrochet in 1842, no one doubted or qualified 

 the statement that if the surface of the water be touched with oil 

 the motions are instantly arrested. Thus Volta says, " If the 

 water be defiled with any foreign substance, or its surface only 

 slightly fouled with oily matter, if only the dust of the room be 

 upon it, the looked-for motions of camphor and of benzoin will 

 not take place, or will be so feeble as to be scarcely sensible." 



In a paper read before the Royal Society about eighteen months 

 agof, one of my conclusions was, that an essential oil may arrest 



* Communicated by the Author. 



t See Philosophical Magazine, vol. xxiv. p. 490. 



