G. Jounstone Stoney on the Penetration of Heat across Layers of Gas. 27 
breeze was blowing, which so facilitated evaporation from the drops* that they 
probably maintained a temperature as low as 10°, a temperature which my 
thermometer reached when J left a damp weed in contact with one side of the 
bulb. 
When the globules of methylated spirit were formed upon a beaker nearly full of 
the same liquid, which was progressively warmed, it was found that, when the air was 
still, there was a particular temperature at which the drops were most persistent. At 
this temperature some lasted for as long as twelve or fourteen seconds. As the 
temperature rose beyond this point, the atmosphere of vapour impeded evaporation 
and the persistence of the drops became less ; but by gently blowing on the surface so 
as to accelerate the evaporation, it was found possible to keep some of them in 
existence for two or three minutes,+ during which time they very slowly dwindled 
away till they were quite small, and then suddenly vanished. When the tempera- 
ture of the beaker full of spirit was allowed to fall below the point above 
referred to, the duration of the drops also became progressively less; but they 
could still be formed, though short-lived, at a temperature a little below that of 
the room. 
From this, and from the circumstance that I succeeded in forming some within 
a bottle of methylated spirit which had been standing open for a while, and within 
which evaporation must have been feeble, it is evident that a drop can be supported 
with but a slight difference of temperature between it and the liquid on which it 
rests. In this respect the spheroidal state on liquids differs from that in which the 
drop rests upon a heated solid. The difference of behaviour is probably due to the 
deformation of its natural spherical shape to which a drop is compelled to submit 
when it rests on a rigid surface. Owing to this constraint the surface-tension over 
the drop will force some parts into closer contact, and, moreover, the vibrations 
which always arise in this case must tend to a similar result. On the other hand, 
when the drop is resting on a liquid, it settles tranquilly into a beautiful concave 
socket that can be seen by looking at the surface of the fluid from beneath. This 
socket allows the globule to retain a nearly spherical and therefore unconstrained 
form, and, accordingly, the opposed surfaces come within an approximately equal 
distance of one another throughout a’ large are. And it is evident that as the 
whole pressure arising from the molecular motions in the air would support a 
column of spirits of wine 114 metres high, it needs only a very moderate Crookes’s 
procession across the stratum of air to furnish that slight preponderance of 
*The liquid on which the drops rest is no doubt also cooled by evaporation, but in a trifling degree, 
because convection currents constantly bring to the surface an accession of warm liquid from below. 
} In three better arranged experiments, since made in the laboratory of the Society by Mr. Moss and 
myself, and of which we hope to give an account to the Society, we succeeded in maintaining similar 
drops formed on ether for ten, fourteen, and sixteen minutes, respectively ; and we believe that it will not 
be difficult, by securing a greater constancy in the conditions indicated by the theory, to prolong their 
existence very much more. 
