614 
Proceedings of the Royal Society 
meter attached thereto, of dimensions determined by actual experi- 
ment. The only source of error to which the instrument seems 
liable is the counteracting influence of dense carbonic acid gas in 
choke-damp. But it is found that the presence of choke-damp 
destroys the explosive character of fire-damp ; and, so far as experi- 
ments go, it seems certain that, in all cases when the presence of 
choke-damp prevents the instrument from indicating the presence of 
fire-damp, the fire-damp is denuded of its explosive character. 
The second instrument is a combination of a harmonium reed 
and an organ pipe, through which the air is driven. They are 
arranged so as to sound the same note when pure air is used, so 
that when there is a lighter gas present the organ pipe sounds a 
higher note, thus producing beats. 
So far as the experiments have gone hitherto, the first form is by 
far the most accurate, being capable of detecting the presence of 1 
or 2 per cent, of fire-damp. 
4. Note on Electrolytic Conduction. By Professor Tait. 
It is commonly said that there is a resistance to a current at the 
surface of contact of a solid conductor and an electrolyte. Some 
good authorities, however, say that we have as yet no proof of this, 
as the effects observed may be due to polarisation. It is obvious 
that, if the reverse electromotive force due to polarisation contain 
a term directly proportional to the strength of the current, the 
ordinary methods of measurement would not enable us to distin- 
guish this from the surface resistance above mentioned. For, in the 
expression 
2(E) 
2(B)’ 
if the numerator contain a term of the form - el, it may be ex- 
punged, provided e be added to the denominator. 
To clear up this point I have recently made a number of experi- 
ments. These have led me to some curious results bearing on the 
theory of electrolysis, which I propose to bring before the Society on 
a future occasion. At present I refer to them merely so far as to 
say that they establish fully the existence of the surface resistance 
above mentioned. Thus I was led to see that if a slip of platinum 
