6 British Association for the Advancement of Science. 



was the detritus from hills, whose rocks were often highly mag- 

 netic. The engineers employed on the trigonometrical survey of 

 Ireland, had erected a mound of stones composed of basalt, to 

 sustain the signal-staff which they had erected on the highest hill, 

 near* Belfast : the effect of that heap of stones on the magnetic 

 needle was so great, that in walking round it, the needle would 

 veer round to every point of the compass. 



Electro-Magnetic Curretits. — M. de la Rive then read a paper 

 ' On the Interference of the Electro-magnetic Currents.' This 

 distinguished foreigner addressed the Section in the French lan- 

 guage. After a brief resume of the known properties of electro- 

 magnetic currents, he adverted to some new results at which he 

 had arrived in studying them. He remarked, that in chemical 

 decomposition effected by these currents, the individual force of 

 each was greater the more rapidly they succeeded each other ; so 

 that, to decompose a given quantity of water, it becomes neces- 

 sary to have a number of these currents, so much the greater as 

 the succession is less rapid. There is, however, a limit, beyond 

 which the force of the currents is not augmented by any further 

 augmentation of the rapidity of the succession. When plates of 

 platina are employed, instead of wires, in the decomposition of 

 water, the decomposition ceases to take place when the surface of 

 contact of the metal with the liquid surpasses a certain limit. 

 Nevertheless, the current, far from diminishing in intensity, be- 

 comes, on the contrary, more intense, — as is shown by the indica- 

 tions of a metallic thermometer, — the helix of which, placed in 

 the current, furnishes a measure of its calorific energy. As soon 

 as the surfaces of contact are of such magnitude that decomposi- 

 tion is no longer effected, the thermometer reaches a maximum, 

 which it does not pass, even when the surfaces of contact are aug- 

 mented. This fact seems to prove, that chemical decomposition 

 produced by electrical currents takes place only when these cur- 

 rents undergo a certain resistance in their passage from the metal 

 into the liquid ; and that, when this resistance does not exist, de- 

 composition ceases. When we employ wires of platina to trans- 

 mit the magneto-electric currents into a solution of any kind, 

 whether acid, saline, or alkaline, we, at first, observe an abundant 

 evolution of gas ; then this disengagement diminishes, and at the 

 end of fifteen or twenty minutes it altogether disappears. When 

 we examine these metallic wires, we find them covered with a 



