302 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES, 
common quill, even when the eighth of an inch in length or more, will show 
the elfect. Chemically pure and finely divided silica rivals lycopodium in the 
beauty of its arrangement at the vibrating parts of the plate, although the same 
substance in sand or heavy particles proceeds to the lines of rest. Peroxide of 
tin, red lead, vermilion, sulphate of baryta, and other heavy powders when 
highly attenuated, collect also at the vibrating parts. Hence it is evident that 
the nature of the powder has nothing to do with its collection at the centres 
of agitation, provided it be dry and fine. 
9. The cause of these effects appeared to me, from the first, to exist in the 
medium within which the vibrating plate and powder were placed, and every 
experiment which I have made, together with all those in M. Savart’s paper, 
either strongly confirm, or agree with this view. When a plate is made to 
vibrate (2), currents (24) are established in the air lying upon the surface of 
the plate, which pass from the quiescent lines towards the centres or lines 
of vibration, that is, towards those parts of the plates where the excursions 
are greatest, and then proceeding outwards from the plate to a greater or 
smaller distance, return towards the quiescent lines. The rapidity of these 
currents, the distance to which they rise from the plate at the centre of oscil- 
lation, or any other part, the blending of the progressing and returning air, 
their power of carrying light or heavy particles, and with more or less rapidity 
or force, are dependent upon the intensity or force of the vibrations, the me- 
dium in which the vibrating plate is placed, the vicinity of the centre of vibra- 
tion to the limit or edge of the plate, and other circumstances, which a simple 
experiment or two will immediately show must exert much influence on the 
phenomena. 
10. So strong and powerful are these currents, that when the vibrations were 
energetic, the plate might be inclined 5°, 6°, or 8° to the horizon and yet the 
gathering clouds retain their places. As the vibrations diminished in force, the 
little heaps formed from the cloud descended the hill ; but on strengthening 
the vibrations they melted away, the particles ascending the inclined plane on 
those sides proceeding upwards, and passing again to the cloud. This took 
place when neither sand nor filings could rest on the quiescent or nodal 
lines. Nothing could remain upon the plate except those particles which were 
so fine as to be governed by the currents, which (if they exist at all) it is evi- 
dent would exist in whatever situation the plate was placed. 
