430 Prof. Gr. Quincke on the Edge-angle and 



evaporation than in the latter case. In the former case the film 

 exhibits a tint of higher order on Newton's scale than in the 

 latter — blue of the 1st order corresponding to a thickness of 

 air of about 0'0002 millim., brown of the 1st order corre- 

 sponding to a thickness of about 0*0001 millim., on the 

 supposition that the thinnest place of the film in reflected light 

 must appear white). 



A drop of alcohol deposited upon a plate, and which more 

 readily spreads (compare § 6 above) into a thicker film, even 

 with a less-pure surface exhibits, like water, by reflected light, 

 the blue of the 1st order. 



The greater or lesser purity, and the greater or lesser 

 capacity of the solid surface to give breath-figures, may 

 therefore be estimated by the greater or lesser readiness with 

 which water spreads, or by the greater or lesser thickness of 

 the film of water which forms on breathing. 



To judge by the measurements communicated in §§ 4-8, in 

 spite of all precautions we shall never attain to the point of 

 giving a clean surface even for a short time only to a solid 

 body, or of keeping such for a longer time. 



And safe as it may be to affirm that upon the surface of a 

 solid body 1 a thin film of foreign matter exists, or, as I will 

 term it, adheres, it must be difficult to determine accurately 

 the nature and chemical properties of this adherent film. At 

 all events by the contact or close proximity of other solids or 

 liquids (porous substances, polished stones, coins) the thick- 

 ness of the adhering film may be diminished ; or it may be 

 increased by the deposition of fresh matter. The places of 

 different thickness are distinguished after breathing upon the 

 surface by differing edge-angles, or different appearance in the 

 breath-figure. 



If a sheet of paper cut into a pattern be laid upon a mo- 

 derately clean surface of glass, and the pattern so cut away be 

 then breathed upon, and the moisture be again allowed to 

 evaporate, the water vapour carries off with it a portion of the 

 adhering film of liquid or gaseous matter, the thickness of the 

 film becomes less, and, on repeating the breathing after the 

 removal of the sheet of paper, the edge-angle on the parts 

 lying underneath the cut-away pattern will be less. The 

 pattern will be visible in the breath-figure. 



Electric currents are, as I have shown in another place*, 

 specially endowed with the property of setting in motion 

 liquids at the surface of solid bodies, even such as were not 

 previously movable. The moving force is, cceteris paribus, 

 the greater as the tension of the current is greater. It cannot, 

 * Pogg. Ann. cxiii. pp. 514 & 592 (1861). 



