380 Mr. C. Tomlinson on the Motions of Camphor 



some flies that had been drowned in the oil. These flies pre- 

 sently began to move, and turned round on the water very ra- 

 pidly as if they were vigorously alive." The experiment was 

 repeated before Dr. Franklin, who says, " To show that it was 

 not any effect of life recovered by the flies, I imitated it by little 

 bits of oiled chips and paper cut in the form of a comma of the 

 size of a common fly, when the stream of repelling particles 

 issuing from the point made the comma turn round the contrary 

 way." The Doctor adds that " this is not a chamber experiment ; 

 for it cannot well be repeated in a bowl or dish of water on a 

 table." The reason for this is want of chemical purity in the 

 bowl or dish. The effect may be well shown in a clean glass 5 

 or 6 inches in diameter, by moulding small coracles of paper on 

 the rounded end of a glass rod, pouring into one of these coracles 

 a few drops of a volatile oil, and placing it on the surface of 

 water. The paper-comma experiment is sometimes referred to 

 as originating with the brothers Weber; but in truth their 

 labours added nothing to the subject, nor does it appear that 

 they intended to do more than glance at it. They do not settle 

 any thing, but, on the contrary, declare that the varied pheno- 

 mena connected with the subject still remain unexplained 18 . 

 They are even inclined to fall back upon an electrical theory in 

 order to explain the rapidity with whieh a drop of oil spreads on 

 the surface of water. 



13. In 1828 August 19 described an experiment by Wirth, in 

 which a metal ball is suspended at a short distance from the 

 surface of water sprinkled over with powdered sealing-wax. On 

 closing the hands over the metal ball the fragments of sealing- 

 wax are set in motion, an effect attributed by Wirth to the mag- 

 netism of the human body, but by August to currents of air due 

 to differences in temperature. He supports this view by show- 

 ing that the effect can be produced by the action of a heated 

 cylinder 20 . 



14. In 1 836 Challis 21 endeavoured to account for the spread- 

 ing of a drop of oil on the surface of water by considering that 

 the angle formed by the free surface of this drop and the com- 



18 They say : — " Die ganze Erscheinung ist noch gar nicht erklart." 



19 Pogg. Ann. S. 2. vol. xiv. p. 42.9. 



20 This class of experiment originated with B. Prevost at the beginning 

 of the present century (Ann. de Chim. vol. xxiv. p. 31 ). These experiments 

 (of which the following is the leading one) are remarkable, and excited a 

 good deal of attention at the time. If near the edge of a disk of tinfoil 

 floating on the surface of water we present obliquely a rod of heated metal, 

 or the focus of solar rays by means of a burning-glass, the disk moves away 

 from the source of heat. This experiment, now so easily explained on the 

 principle of surface-tension, led Prevost into some very elaborate specu- 

 lations. 



21 Phil. Mag. vol. viii. p. 288. 



