366 



Motions of Camphor vpon Water. 



[Mar. 27, 



nience of comparison they are arranged, not in order of date, but in 

 order of densities of film. 



The sharpest test of the quantity of oil appeared to occur when the 

 motions were nearly, but not quite, stopped. There may be some 

 little uncertainty as to the precise standard indicated by " nearly 

 enough," and it may have varied slightly upon different occasions. 

 But the results are quite distinct, and under the circumstances very 

 accordant. The thickness of oil required to take the life out of the 

 camphor movements lies between one and two millionths of a milli- 

 metre, and may be estimated with some precision at 1*6 micro- 

 millimetre. Preliminary results from a water surface of less area 

 are quite in harmony. 



For purposes of comparison it will be interesting to note that the 



A Sample of Oil, somewhat decolorised by exposure. 



Date. 



Weight of 

 oil. 



Calculated 

 thickness 

 of film. 



Effect upon camphor fragments. 



Dec. 17 . . . 

 Jan. 11 . . 

 Jan. 14. . . 

 Dec. 20 . . . 

 Jan. 11 . . . 

 Dec. 17 . . 

 Dec. 18 . . . 

 Jan. 22 . . . 

 Dec. 18 . . . 

 Dec. 17 . . . 

 Dec. 20 . . . 



-40 mg. 

 -52 

 0-65 

 0-78 

 0-78* 

 0-81 

 83 

 -84 

 0-95 



0- 99 



1- 31 



0- 81 

 1 06 



1- 32 

 1 '58 

 158 

 1-63 



1 -68 

 1*70 



1- 92 



2- 00 



2 65 



No distinct effect. 

 Barely perceptible. 

 Not quite enough. 

 Nearly enough. 

 Just enough. 

 Just about enough. 

 Nearly enough. 

 About enough. 

 Just enough. 



All movements very nearly stopped. 

 Fully enough. 



A fresh Sample. 



Jan. 28 . . . 

 Jan. 28 . . . 



0- 63 



1- 06 



1- 28 



2- 14 



Barely perceptible. 

 Just enough. 



thickness of the black parts of soap films was found by Messrs. 

 Remold and Riicker to be 12 micro-millimetres. 



An important question presents itself as to how far, these water 

 surfaces may be supposed to have been clean to begin with. I believe 

 that all ordinary water surfaces are sensibly contaminated ; but the 

 agreement of the results in the Table seems to render it probable that 

 the initial film was not comparable with that purposely contributed. 

 Indeed, the difficulties of the experiments proved to be less than had 

 been expected. Even a twenty-four hours' exposure to the air of the 



