84 PROFESSOR BARRETT, 
vapour, and partly on the proximity of the two surfaces. <A 
proximity not to be estimated absolutely, but with reference to 
the length to which a molecule of the gas will travel in the 
intervals between its encounters with other molecules. Hence 
there are obviously three modes whereby the excess of pressure, 
this Crookes’ force, may be developed or augmented :— 
1st. Bylengthening the paths of the molecules between the warm _ 
and cool surfaces, accomplished by attenuating the gas. 
2nd. By bringing the hot and cold surfaces very near together. 
3rd. By increasing the difference of temperature between the 
two surfaces. 
Now if the support of the spheroidal drop be due to this 
Crookes’ force a difference of temperature must exist between the 
drop and the surface over which it stands, and the greater this 
difference of temperature the larger the drop that ought to be 
supported, and the more persistent the phenomenon. Mr. Moss 
has shown (Proc. R. D.8., Dec. 1877) that by securing a continual 
difference of temperature a globule of ether may be-supported on 
the surface of its own liquid for upwards of an hour, until in 
fact some accidental derangement occurs. The conditions of the 
two theories being thus defined, it is easy to see that several 
crucial experiments might be devised which should help to 
decide the question at issue. 
The following experiment the author has made with this object 
in view. Uponthe surface of the ordinary petroleum of commerce, 
liquid globules of transient duration can readily be formed, simply 
by removing a small quantity of the liquid in a pipette and care- 
fully depositing a drop on the surface of the liquid. These drops 
are clearly in the spheroidal condition, and they are easily and 
abundantly formed by dipping a vibrating tuning fork into the 
liquid, or by drawing a fiddle bow over the edge of the vessel 
containing the liquid. According to the ordinary explanation 
the drops are supported by the elastic force of the vapour of the 
liquid, which would, of course, be greater the higher the tempera- 
ture of both liquid and drops. According to Mr. Stoney’s theory 
the drops are supported by the Crookes’ force, generated by the 
proximity of the drop and liquid, and by the fact that they are 
at different temperatures. Evaporation rapidly cools the drops 
jerked up from the liquid, and thusa slight difference of tempera- 
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