191G] 



on Problems in Capillarity 



'87 



cblor-benzene, tetra-hydro-quinoline and olive oil are a set of bodies 

 tliat show marked contrasts. 



Solid C'ollodion Film^. 



A sobition of nitrated cotton can be readily made in amyl 

 acetate. If a drop of this solution is allowed to touch the water 



surface, a circular film showing the rings of colour steadily spreads 

 out. The amyl acetate quickly evaporates, as shown l)y the regular 

 disappearance of the colours, and an extremely thin black solid film 

 of tlie nitro-cotton is left. This can be folded over, showing the 

 stages of colour produced by increasing thickness. If a flat ring 

 support is carefully slipped under the film it can, by careful 

 handling, be completely removed by a drawing motion at a steadily 

 increasing elevation. Black films 15 cm. in diameter can be so 

 obtained. They are easily dried, and quite permanent with careful 

 handling. They can be weighed, and their thickness thereby esti- 

 mated. For example, the mean weight of several films 11 cm. 

 in diameter was found to be 0'432 mgms. ; the density of the 

 collodion cotton being 1 • GO, it follows that the thickness of the black 

 film, if uniform, will be of the order of 3 x 10" '^ cm., i.e. 30 /x/x. The 

 average thickness of a film can jdso l)e found by knowing the weight 

 of a drop of a collodion solution in a known concentration, and 



3 F 2 



