365 
of Edinburgh , Session 1879-80. 
conclude that a chemically pure surface of these metals is impossible 
for more than a very few seconds after cleaning, even if for so long. 
These experiments are to he repeated with the aid of an improved 
method of effecting contact. 
3. On an Instrument for detecting Coal-Gas in Mines. 
By Professor George Forbes. 
In 1877, shortly after the disastrous colliery explosion in the 
Blantyre pit, in which hundreds of lives were lost, Mr James Young, 
F.R.S., of Kelly, described to me an instrument which he had 
thought of for determining what is the amount of fire-damp in any part 
of a mine. This was the first thing which directed my attention to 
the subject, and I very soon saw that there was a principle in acoustics 
which might be most admirably adapted to the end in view, viz., to 
determine the quantity of fire-damp (or marsh gas) by the diminution 
in density of the mixed air and gas (for marsh gas is only about half 
the density of air). Mr Young and myself tested the principle the 
next day, and found it to be one of extreme delicacy. I then, in the 
spring of 1878, communicated to this Society the principle which I 
proposed to utilise in the form of a preliminary note. I have now 
the honour of exhibiting the instrument, which has been completed 
and perfected, partly by my own labours and partly by the appoint- 
ment, for the purpose, of a Committee of the British Association, 
consisting of Professor W. J. Adams, Professor Ayrton, and myself. 
The form of instrument finally adopted is one in which a tuning- 
fork is set into vibration by drawing through between the prongs a 
tight-fitting piece of metal. Just under the points of the prongs a 
tube 1J inch diameter is fixed. The lower end of this tube is closed 
by a tight-fitting piston, whose position in the tube can be altered so 
as to regulate the length of the closed tube. 
It is a well-known principle in acoustics that when a vibrating 
tuning-fork is so held over a tube, the air in the tube will resound 
and intensify the sound when the tube has a certain definite length. 
Moreover, this length depends on the kind of air or gas with which 
the tube is filled, being longer for a heavy gas, and shorter for a light 
gas, at the same pressure. Now a mixture of air and marsh gas is 
lighter than pure air in proportion as the dilution with marsh gas is 
