size. Thus butyric acid which lias sensibly the same specific 
gravity as water, gives rise to a drop of less than half the size 
of the water drop: while mercury of singular specific gravity 
lias no exceptional drop-size. Lastly it may be observed how 
that remarkable body, wa f er asserts here again its pre-emi- 
nence. 'flic first impression which these numbers make is 
that there is three groups of magnitude n, 2n, 3n. But it is 
possible that a change in the nature of the solid might throw 
these drop-sizes into a different order; and certainly until 
a very much greater number of bodies is examined in this 
sense it would be premature to attempt to establish anything 
like a law. 
It is sufficient for the present to point out that the drop- 
size is not directly dependent upon either the specific gravity 
or boiling point. Xor does it stand in any obvious relation 
to what is sometimes called the liquidity, mobility or “ thin- 
ness ” of a liquid. For we find that glycerine and (from for- 
mer experiments) cocoa -nut-oil both from smaller drops than 
water, the one being heavier and the other lighter than that 
body ; and both being sluggish. On the other hand alcohol 
and acetic acid both perfectly mobile liquids give rise to drops 
about half as large as those of glycerine.* 
Hence it is clear that we are still ignorant of that property 
of a liquid which determines its drop-size, and arc not yet in 
a position to connect the drop-size with any of the known 
physical or chemical properties of liquids. W e approach the 
solution of the problem by studying the effects of change in 
some others of the variables. 
The adhesion between the liquid which drops and the solid 
from which it drops is also affected by the curvature and ge- 
neral geometric distribution of the solid at and about its low- 
* Tie evaporation of the more volatile of these liquids is a source of 
slight error ; not so much on account of t he direct, loss by weight of the 
drop in falling, as by reason of the cooling which it causes and the con 
sequent variation in density and adhesion. Such source rt variation 
■will form a separate subject ot investigation. 
