MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 309 
division, or upon any immediate and peculiar action of the plate, but upon the 
currents of air necessarily formed over its surface, in consequence of the extra- 
mechanical action of one part beyond another. In this point of view the nature 
of the medium in which those currents were formed ought to have great influence 
over the phenomena ; for the only reason why silica as sand should pass towards 
the quiescent lines, whilst the same silica as fine powder went from them, is, that 
in its first form the particles are thrown up so high by the vibrations as to be 
above the currents, and that if they were not thus thrown out of their reach they 
would be too heavy to be governed by them ; whilst in the second form they are 
not thrown out of the lower current, except near the principal place of oscillation, 
and are so light as to be carried by it in whatever direction it may proceed. 
33. In the exhausted receiver of the air-pump therefore the phenomena 
ought not to occur as in air ; for as the force of the currents would be there 
excessively weakened, the light powders ought to assume the part of heavier 
grains in the air. Again, in denser media than air, as in water for instance, 
there was every reason to expect that the heavier powder, as sand and filings, 
would perform the part of light powders in air, and be carried from the qui- 
escent to the vibrating parts. 
34. The experiments in the air-pump receiver were made in two ways. A 
round plate of glass was supported on four narrow cork legs upon a table, and 
then a thin glass rod with a rounded end held perpendicularly upon the middle 
of the glass. By passing the moistened fingers longitudinally along this rod the 
plate was thrown into a vibratory state ; the cork legs were then adjusted in 
the circular nodal line occurring with this mode of vibration; and when their 
places were thus found they were permanently fixed. The plate was then trans- 
ferred into the receiver of an air-pump, and the glass rod by which it was to 
be thrown into vibration passed through collars in the upper part of the 
receiver, the entrance of air there being prevented by abundance of pomatum. 
When fine silica was sprinkled upon the plate, and the plate vibrated by the 
wet fingers applied to the rod, the receiver not being exhausted, the fine 
powder travelled from the nodal line, part collecting at the centre, and other 
part in a circle, between the nodal line and the edge. Both these situations 
were places of vibration, and exhibited themselves as such by the agitation of 
the powder. Upon again sprinkling fine silica uniformly over the plate, ex- 
