— 196 
the size of the broken off part or drop depends in great mea- 
sure upon the thickness of the residual film, as we have seen 
in examining the influence of the growth time, and the radius 
of curvature. 
Adhesion may also exist between a solid and a liquid which 
does not wet it, as when a drop of mercury hangs from a glass 
sphere. 13 ut the cohesion of the liquid in such a case, by its 
effort to bring the liquid to the spherical form, and the weight 
of the drop so modify the adhesion between the solid and li- 
quid by altering the size of the surface of contract between 
the two, that the size of the drop gives no direct clue to the 
cohesion of the liquid. 
We may now examine a few cases in which, the size of the 
sphere remaining the same and its density in some instances 
nearly so, the matter of the solid varies ; the liquid however 
wetting it in all cases. This Avill show whether the differences 
of Table XI are due wholly to differences of density of the 
solid or also or wholly to differences of chemical constitution. 
Equal spheres of the substances were made by casting them 
in the same bullet mould. The surfaces of the metals were 
roughened by momentary immersion in acid : Tin and Anti- 
mony in Hydrochloric and the others in Nitric acid. Without 
this precaution a metallic surface is apt to be wetted only lo- 
cally, the base edge of the drop is irregular and inconstant 
and the drop -weight varies. Indeed with some metals such 
as Tin, a smooth and bright surface is scarcely wetted by water. 
As the bodies examined have different coefficients of ex- 
pansion by heat ; and one of them expands on solidification, 
it was necessary to test the equality of their size and remedy 
any inequality. This was done by arranging three of them 
one at each corner of small equilateral triangle drawn on a 
large piece of plate glass. Another piece of plate glass was 
then placed upon the spheres, so as to rest on all three and was 
slightly loaded. On passing a gauge between the plates at 
their edges, the slightest inequality of the spheres could 
be detected because the gauge lifted the plate off the smallest 
