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 Currents.—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- 
ter a brief résumé 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 mnnatad 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 suécession.— When plates of 
platina are employed, instead of wires, in the deco 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 atig- 
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 dininuageincts diminishes, and at the 
end of fifteen or twenty minutes it altogether disappears. When 
we examine these metallic wires, we find them covered with & 
ee ae 
aoe eerie? 
