28  G. Jonnstone Stoney on the Penetration of Heat across Layers of Gas. 
velocities in a determinate direction, which is required to support the few 
additional millimetres that correspond to the weight of the drop. We must 
remember, too, that a drop may, under favourable circumstances, outlive the 
difference of temperature, because it would take a sensible time for so slight a 
pressure as the weight of the drop to squeeze the film of air, when once established, 
out of its narrow chink. Even a heavy metal proof-plane will float over another’ 
proof-plane upon the stratum of air entangled between them, for a considerable 
time. 
22, Professor Barrett has called my attention to another unexplained phenomenon, 
of which we can now see the cause, viz., the mobility imparted to a very fine 
powder, as, for example, magnesium carbonate or precipitated silica, by heating it 
in a metal dish. When the dish is disturbed the powder glides about as if floating ; 
and it is in fact floating on the compressed Crookes’s layer, which will spring into 
existence whenever the powder is able by radiation to maintain a lower temperature 
than the dish. ; 
23. The communication of heat by penetration is a very familiar phenomenon, 
for when surfaces at different temperatures are brought into what is commonly 
called contact, there is usually a thin intervening stratum of air, except at special 
points; and, accordingly, the greater part of the transfer of heat, so long as the 
difference of temperature is considerable, must be effected, not by contact, but by 
penetration across a Crookes’s layer. 
/ 
Nore to Section 8.—De la Provostaye and Desains conclude their second memoir in the following 
words :—“ Nous ne chercherons 4 donner une explication compléte des différents faits cités dans cette 
communication. Nous ferons remarquer seulement que le pouvoir refroidissant d’un gaz dépend de sa 
densité et de sa mobilité. Ces deux éléments varient en sens inverse quand on change la pression, et 
Yon concgoit que les effets de ces variations contraires puissent tantdt s’équilibrer, tantot se surpasser dans 
un sens ou dans |’autre.” 
Nore ro Sections 10, 12, and 15.—An account of De la Provostaye and Desains’ experiments is 
given also in the Annales de Chimie, Third Series, Vol, xvi., p. 381, and Vol. xxii, p. 362. This 
account does not differ much from the report in the Comptes Rendus, but contains one important addition, 
viz.—a statement of the size and shape of the thermometer bulb. It was a cylinder 7 cm. long and, 
2 cm. across; so that the interval between the bulb and the wall of the smallest receiver was 2 cm. 
The reader is accordingly requested to substitute 2 cm. for 14 cm. in Sections 10, 12, and 15. 
