292 Messrs. Scott and Galloway on the Connexion [Apr. 18^ 



copper to silver, that is, in the same direction as if the copper had been 

 dissolved by an acid and hydrogen evolved on the silver plate. 



If the two plates have their sides parallel, the suboxide is deposited not 

 merely on that side of the silver plate which faces the copper, but after 

 about a minute on the other side also, showing that in this, as in other 

 cases, the lines of force curve round. 



It became interesting to consider what started this electric current. The 

 original observations convinced us that it was not due to the action of oxygen 

 on the copper ; but, to make the matter more certain, bright copper aud 

 silver plates in conjunction were immersed, the copper in a pure, i. e. deoxy- 

 genized, solution of nitrate of copper, the silver in an oxygenized solution : 

 the two liquids communicated through the diaphragm of a divided cell. 

 In half an hour the silver plate was covered with a reddish film, while not 

 a trace of tarnish was perceptible on the copper. On continuing this 

 experiment for three hours, it was found that the copper plate lost 

 0"003 grm., and the silver plate was increased by 0'004 grm. On cleaning 

 the plates, and reversing their position, the copper was covered with a film 

 of oxide, while the silver remained free from cupreous deposit. We believe 

 therefore that, through the simultaneous action of the two metals, the 

 dissolved salt is put into such a state of tension that oxygen brings about a 

 chemical change which otherwise would be impossible, and that this change 

 is initiated in close proximity to the more negative metal. 



Though we have examined only this particular reaction, we have satis- 

 fied ourselves that it is not an isolated fact. Each of the elements con- 

 cerned may be replaced by others : thus, the sulphate may be substituted 

 for the nitrate of copper, or platinum may be used instead of silver; 

 chlorine may take the place of oxygen, with the production of the sub- 

 chloride instead of the suboxide ; and zinc may be employed as the positive 

 metal, with zinc chloride as the salt in solution, in which case copper may 

 be taken as the negative metal, and on its surface will form a deposit of 

 oxide of zinc. 



April 18, 1872. 



WILLIAM SPOTTISWOODE, M.A,, Treasurer and Vice-President, 



in the Chair. 



The following communications were read : — 



I. " On the Connexion between Explosions in Collieries and Weather." 

 By Robert H. Scott, M.A., F.R.S., and William Galloway, 

 Esq., Mining Engineer. Received March 19, 1872. 



The gas commonly called "Fire-damp," to the mixture of which with 

 atmospherical air the formation of the explosive mixture in coal-mines is 

 due, exudes from the coal at a certain pressure ; so that the rate of its 

 escape must, to some extent, depend on the pressure of the atmosphere, 



