of Edinburgh, Session 1879-80. 701 
simplest and most certain effect of heat passed over. I must confess 
that the communication deserved that criticism, for the possibility of 
longitudinal extensions and contractions is not once referred to. At 
the same time I may say that I thought then, as I do now, that such 
extensions and contractions are not the cause of these sounds, and 
the object of this note is to give the ground for such a belief. I 
may shortly recapitulate why I thought so then. 
In the first place I was satisfied with the account given by De la 
Rive. He looked upon the sounds as a magnetic phenomenon. The 
wire became somehow magnetised and demagnetised by the beginning 
and end of the current, and the molecules of the wire, in taking and 
losing the magnetic set, hit against each other and emitted the sounds 
in question. He thought that such was the case from the exceptional 
position that iron occupied among the metals he worked with, and 
from the exactly similar action of wires within a magnetising spiral 
through which an intermittent current was sent, and those giving 
direct passage to the same current. The Bell telephone gave me an 
additional confirmation of this view. Any one who listens to the 
sounds emitted by discs of different metals when the telephone is 
excited by a strong discontinuous current and then listens to those 
given out by wires of the same metals when excited by direct pas- 
sage of the same current, as revealed by the thread telephone, cannot 
fail to be struck by the perfect correspondence of the results in both 
cases. If parity of performance can give any ground for suspecting 
the same cause, then if the action of the Bell discs be attributed to 
magnetism, so must also be the action of these sounding wires. This 
seemed to me at the time convincing enough. 
But in addition to De la Rive’s magnetic theory, it seemed to 
me that the sounds originated within the wires, and that they did 
not need to expend their blows on anything external before sounds 
were produced. They could be heard when the wires were lying 
loosely on a table by the aid of a telephone with a wire thread 
soldered to them. When thrust into the passage of the ear the 
wires could be heard distinctly without any device for magnifying 
their loudness. An arc of wire suspended from the thread of a 
telephone, with each end dipping into adjoining cups of mercury, 
sounded as loud as if abutting on something solid. Again, it 
seemed to me unlikely that a wire should receive and divest itself 
