the Weights of Atoms. 181 



absolute demonstration that the film retains its tensile strength 

 in the black spot " where the thickness is clearly much less 

 " than 1 /60000 of a centimetre, this being the thickness of 

 '•'the dusky white" with which the black spot is bordered. 

 And further in 1883 Reinold and Rilcker's * admirable 

 application of optical and electrical methods of measurement 

 proved that the thickness of the black film in Plateau's 

 '" liquide glycerique " and in ordinary soap solution is between 

 one eight-hundred- thousandth of a centimetre and one mil- 

 lionth of a centimetre. Thus it was certain that the soap- 

 film has full tensile strength at a thickness of about a 

 millionth of a centimetre, and that between one millionth 

 and one one-hundred-millionth the tensile strength falls off 

 enormously. 



§ 28. Extremely interesting in connection with this is the 

 investigation, carried on independently by Rontgen f and 

 Rayleigh J, and published by each in 1890, of the quantity- 

 of oil spreading over water per unit area required to produce 

 a sensible disturbance of its capillary tension. Both experi- 

 menters expressed results in terms of thickness of the film, 

 calculated as if oil were infinitely homogeneous and therefore 

 structureless, but with very distinct reference to the certainty 

 that their films were molecular structures not approximately 

 homogeneous. Rayleigh found that olive oil, spreading out 

 rapidly all round on a previously cleaned surface of water 

 from a little store carried by a short length of platinum wire, 

 produced a perceptible effect on little floating fragments of 

 camphor at places where the thickness of the oil was 

 10*6 x 10 -8 cm., and no perceptible effect where the thickness 

 was 81xl0 _s cm. It will be highly interesting to find, if 

 possible, other tests (optical or dynamical or electrical or 

 chemical) for the presence of a film of oil over water, or of 

 films of various liquids over solids such as glass or metals, 

 demonstrating by definite effects smaller and smaller thick- 

 nesses. Rontgen, using ether instead of camphor, found 

 analogous evidence of layers 5*6 x 10~ 8 cm. thick. It will 

 be very interesting for example to make a thorough invest- 

 igation of the electric conductance of a clean rod of white 

 glass of highest insulating quality surrounded by an atmo- 

 sphere containing measured quantities of vapour of water. 

 AVhen the glass is at any temperature above the dew-point of 



* " On the Limiting Thickness of Liquid Films,*' Eov. Soc. Proc. 

 April 19, 1883 ; Phil. Trans. 1883, part ii. p. 645. 

 t Wied. Ann. vol. xli. 1890, p. 321. 

 X Proc. Ptov. Soc. vol. xlvii. 1890, p. 3G4 



