MR. FARADAY ON THE GROUPING OF PARTICLES, &C. 
315 
centres or lines of vibration of plates, membranes, &c. under common circum- 
stances ; and that no secondary mode of division need be assumed to account 
for them. I have been the more desirous of accumulating- experimental evi- 
dence, because I have thought on the one hand that the authority of Savart 
should not be doubted on slight grounds, and on the other, that if by accident 
it be placed in the wrong scale, the weight of evidence against it should be 
such as fully to establish the truth and prevent a repetition of the error by 
others. 
52. It must be evident that the phenomena of collection at the centres or 
lines of greatest vibration are exhibited in their purest form at those places 
which are surrounded by nodal lines ; and that where the centre or place of 
vibration is at or near to an edge, the effects must be very much modified by 
the manner in which the air is there agitated. It is this influence, which, in 
the square plates (6. 12) and other arrangements, prevents the clouds being at 
the very edge of the glass. They may be well illustrated by vibrating tin 
plates under water over a white bottom, and sprinkling dark-coloured sand or 
filings upon various parts of the plates. 
On the 'peculiar Arrangement and Motions of the heaps formed by particles 
lying on vibrating surfaces. 
53. The peculiar manner in which the fine powder upon a vibrating surface 
is accumulated into little heaps, either hemispherical or merely rounded, and 
larger or smaller in size, has already been described (6. 28), as well also as the 
singular motion which they possess, as long as the plate continues in vibra- 
tion. These heaps form on any part of the surface which is in a vibratory 
state, and not merely under the clouds produced at the centres of vibration, 
although the particles of the clouds always settle into similar heaps. They 
have a tendency, as heaps, to proceed to the nodal or quiescent lines, but are 
often swept away in powder by the currents already described (6). When on 
a place of rest, they do not acquire the involving motion. When two or more 
are near together or touch, they will frequently coalesce and form but one 
heap, which quickly acquires a rounded outline. When in their most perfect 
and final form, they are always round. 
54. The moving heaps formed by lycopodium on large stretched drawing- 
