Spread of Liquids on Solid Bodies. 429 



of silver of various thickness upon glass were, amongst others, 

 measured. 



The substance 2 of the thin film upon the surface of the 

 solid body lmay therefore consist, as experience teaches, either 

 of solid or liquid. 



That it may also consist of gaseous matter or condensed 

 gas cannot be doubted, since I have completely established 

 the influence of electrolytically separated films of gas on the 

 bounding surface of mercury and water, or of various acids 

 and saline solutions*. 



Consequently, the surface 1 may belong to a solid body or 

 to a liquid, and the thin film may consist of solid, liquid, or 

 gaseous matter. 



If the vapour of a liquid 3 be allowed to deposit upon the 

 cooler surface 1 of a body rendered impure by a thin film 2, 

 the condensed drops possess an edge-angle varying according 

 to the thickness of the film 2 ; the places where the edge- 

 angles have different values reflect the light in different ways ; 

 and thus the various thicknesses of the film 2 can be estimated 

 indirectly. 



When aqueous vapour settles upon a plate of glass or metal, 

 we obtain a so-called breath-figure, such as have been investi- 

 gated principally by Moserf and WaideleJ. If mercury be 

 deposited upon a film of iodide or bromide of silver modified 

 by light or exposure in a camera, we have a so-called photo- 

 graphic image (" light-figure ") or Daguerreotype, a breath- 

 figure produced by vapour of mercury. 



In order that the breath-figures may appear plainly, the 

 thickness of the film 2 must be less than the maximum thick- 

 ness D mentioned above. 



For the production of the breath-figures it is therefore 

 advantageous to employ the cleanest surface possible of the 

 body 1. Surfaces of solids must therefore be freed as much 

 as possible from the adhering film of substance 2 by polishing 

 with alcohol, tripoli-powder, &c. 



Waidele found that freshly polished surfaces of silver gave 

 the breath-figures best when they condensed the moisture 

 with a blue tint, and worse when with a brown tint. This 

 blue or brown tint I observed on the thin film of water which 

 spreads near a drop of water upon a freshly cleansed surface 

 of glass or silver. In the former case a portion of the drop 

 of water spreads in a thicker film upon the solid surface before 

 the liquid on the outer edge of the film has disappeared by 



* Pogg. Ann. cliii. p. 193 (1874). 

 t Ibid, lvi. p. 177 ; lvii. p. 1 (1842). 

 \ Ibid. lix. p. 255 (1843). 



