1879.] 



The Contact Theory of Voltaic Action. 



421 



definitely extended by the mixture of air and coal-dust produced 

 by the disturbance which it initiates. 



The dangers due to the presence of coal-dust in dry mines can be 

 very easily avoided by sprinkling water plentifully on the principal 

 roadways along which the air currents pass, in going to, and coming 

 from, the working places. For example, Llwynypia Colliery, which 

 was formerly one of the driest and most dusty of the mines in the South 

 Wales basin, is now kept constantly damp or wet in this way with a 

 daily expenditure of about 1,800 gallons of water. The amount of 

 air passing through it at present is over 80,000 cubic feet per minute, 

 and its out-put of coal is, on the average, about 800 tons per day. 



[II. " The Contact Theory of Voltaic Action." No. III. By 

 Professors W. E. Ayrton and John Perry. Communi- 

 cated by Dr. C. W. Siemens, F.R.S. Received February 19. 

 1879. 



(Abstract.) 



The authors commence by referring to the experiments that had 

 been made prior to 1876, on the difference of potentials of a solid in 

 contact with a liquid, and of two liquids in contact with one another, 

 and they point out that : — 



1. The earlier experiments were not carried out with apparatus sus- 

 ceptible of giving accurate results. 



2. Owing to the incompleteness of the apparatus assumptions had 

 to be made not justified by the experiments. 



3. No direct experiments had been performed to determine the 

 difference of potential of two liquids in contact, with the exception of 

 a few by Kohlrausch, using a method which appeared to the authors 

 quite inadmissible as regards accuracy of result. 



In consequence of this great vagueness existed as to whether the con- 

 tact difference of potentials between two substances, when one or both 

 were liquids, was a constant depending only on the substances and the 

 temperature, or whether it was a variable dependent upon what other 

 substance was in contact with either. Some authorities regarded it as 

 a variable. Gerland considered he had proved it to be a constant, but 

 first, the agreement of the value of the electromotive force of each of 

 his cells with the algebraical sum of the separate differences of 

 potential at the various surfaces of separation, and which was the test 

 of the accuracy of his theory, was so striking, and so much greater 

 than polarisation, &c, usually allows one to obtain in experiments of 

 such delicacy, that one could not help feeling doubtful regarding his 



