382 Mr. C. Tomlinson on the Motions of Camphor 



nied by volleys of minute globules which extend to the edge of 

 the water in radial lines. Other similar cases of essential oils 

 might be cited ; and even in the case of solid fats the evidence 

 of a thin invisible film covering the surface and arresting the 

 camphor motions, seemed to be irresistible before the promulga- 

 tion of the surface-tension theory. A stick of common mottled 

 soap, or of tallow or lard, lowered into a large surface of water 

 previously dusted over lightly with lycopodium powder, instantly 

 clears away a large circular space (extending nearly to the edge 

 of the glass). This effect is much more satisfactorily explained 

 on the surface-tension theory than on the existence of a thin 

 invisible film. But in the case of oils, whether volatile or fixed, 

 it must be considered that a much greater tensile force is ex- 

 erted in spreading some oils than others. When a film formed by 

 a drop of a volatile oil nearly covers the surface of the water, 

 there is still a sufficient residual amount of surface-tension to 

 cause fragments of camphor to perform their evolutions ; they 

 skate through the film, cutting it up in all directions. But in 

 the case of a film formed by so large a quantity as a drop of any 

 fixed oil, the camphor fragments are motionless, whether on the 

 film itself or on the adjacent water- surface. This was formerly 

 supposed to prove the existence of a fatty film which, though 

 invisible, destroyed contact between the water and the camphor; 

 but now, in accordance with the new theory, the fatty-oil film 

 so far reduces the tension as to leave no residual force sufficient 

 to give motion to the fragments. 



16. But if, as I have shown, a freshly distilled volatile oil 

 does not prevent the motion of the camphor fragments, some 

 deductions must be made from the statements of the earlier ob- 

 servers, which savour of the marvellous. Thus Volta, writing 

 in 1787, says : — " If the water be defiled with any foreign sub- 

 stance, or its surface only slightly fouled with oily matter, if 

 only the dust of the room or of one's clothes 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 24 . So also 

 Prevost 25 , writing in 1797, remarks that " if the surface of the 

 water be touched with a pin previously dipped in oil, the motion 

 of the camphor fragments instantly ceases, just as if they had 

 been struck by lightning (comme foudroyees)." Venturi 26 also, 

 in 1797, says, " Touch the surface with oil, and an almost im- 

 perceptible film instantly advances over the whole surface, repels 

 the fragments, and strikes them motionless as if by magic." 



24 Volta's Latin letter to Frank is contained in the Delectus Opusculo- 

 rum Medicorum. Ticini, 1787. An English translation of this letter is 

 given in my ' Experimental Essays/ See note 8 . 



25 Ann. de Chim. vol. xxi. p. 254. 26 Ibid. p. 262. 



