422 Prof. G. Quincke on the Edge-angle and 



of the liquid will be the greater as the whole film of liquid 

 which covers the solid body is thinner. 



But since the change of density is propagated from particle 

 to particle, it may very well be observable, even at a distance, 

 > I, or greater than pi, where p may signify a factor greater 

 than 2. 



These changes of the density and molecular properties of 

 the liquid, which in the so-called imperceptibly thick films 

 that overspread the surfaces of solid bodies, play a considerable 

 part in all phenomena concerning the attraction of fluids to 

 other fluids or solid bodies, and make the investigation of 

 these phenomena the more difficult, since we know very little 

 or nothing of the magnitude and nature of the changes, and 

 consequently cannot bring them into our calculations. 



The phenomenon not theoretically explicable, that a drop of 

 oil upon a water-surface covered with a thin oil-film, or a 

 drop of water upon a glass-surface covered with a thin water- 

 film remains in a lenticular form, depends probably upon some 

 such modification of the liquid in the thin liquid film. 



Whether the difficult mobility of a lenticular drop of liquid 

 on a level solid surface (compare § 4) is also determined by 

 the presence of an imperceptibly thin film of liquid, as I 

 formerly * observed in the case of liquid surfaces, or whether 

 the difficult mobility of the solid substratum is also involved, 

 cannot be decided with certainty off-hand. The fact that 

 upon the cleanest possible solid surfaces the liquids can spread 

 with remarkable rapidity points to the former suggestion. 



These thin films of liquid, modified by the proximity of a 

 heterogeneous substance, play a great part in nature; and there 

 is certainly no doubt that the life of organic nature depends 

 principally upon them. 



That these films, whose thickness in many cases amounts 

 only to a small fraction of the mean length of a w r ave of light, 

 cannot be seen, is a physical difficulty for investigation, but 

 no proof against their existence. In the investigation of the 

 common surface of different liquids I have already remarked f 

 upon this circumstance, ^nd hinted at the difficulties involved 

 in the investigation. 



But it is possible to prove the existence of these thin films 

 in another way than by the edge-angle at the boundary of a 

 drop of liquid on. a level solid body. Newton's rings were 



* Pogg. Ann. cxxxix. p. 71 (1870). Compare also Marangoni, Cimento, 

 v. p. 239 (1872) ; Berol. Ber. xxviii. p. 184 (1872). 



t Pogg. Ann. cxxxix. pp. 37, 39, 69, 73-76 (1870) ; and Phil. Mag. [IV.] 

 vol. xli. (1871). 



