Many of our readers may have wondered why tall buildings such as church steeples and factory chimneys are provided with thin rods of iron running down their sides; and may have been at a loss to understand their meaning. Their use is to conduct lightning harmlessly to the ground during thunder-storms. We have, however, had warnings enough that a bad lightning-conductor is worse, as regards the security of the building it is supposed to protect, than none at all. Unless the electrical connection with the earth be perfect, the conductor may invite the very danger which it ought to turn aside. Rusted chains, imperfect fittings, and the absence of a sufficient thickness of untarnished metal, are responsible for much mischief. Lightning, properly dealt with, is robbed of much of its terrific power; but when its natural path is blocked, and its swift circuit interrupted, it inevitably rends and tears and burns, scathing and scattering all substances before its resistless might.

Franklin meant the lightning-conductors which he invented to consist of iron alone. Iron, however, has too strong an affinity for oxygen to allow of this. All moisture, and all heat, corrode it more or less; and thus grew up the custom of pointing the conductors with copper, and in some cases with costly platinum, soldered to the iron rod. But exposure to weather, and the weak galvanic currents which unavoidably set in where metal of one sort is in contact with metal of another sort, cause rapid decomposition at the joint, and encourage the rust to eat into the substance of the rod. A heavy flash will melt or cripple a conductor thus imperfect, and then woe to the structure! This defect can now be cured by coating the iron rod completely with nickel, a metal which defies rust, and which conducts electricity better than the pure iron does. Bars and rods of this nickelised iron have been kept under water for several days without tarnishing, and resist the effects of the most powerful battery of Leyden jars.

It had been believed, until lately, that platinum was a metal with which no rogue, however dexterous, could tamper. The platinum coinage of the Russia of thirty years since was considered un-imitable by the manufacturers of false money; while the capsules, crucibles, and other apparatus required by scientific men were sold according to the high market value of what is really a precious metal. Unluckily, fraud has been found possible even in this case. The Director of the Royal Italian Observatory on Vesuvius, M. de Luca, surprised at finding first one and then another of the platinum points of his conductors melted by the effect of lightning, made a careful investigation, and discovered that the platinum had been adulterated with from ten to twelve per cent. of lead, and thus rendered fusible. Platinum thus mixed with an inferior metal can be identified by its lesser density, or more easily by the blowpipe, before which a tell-tale green flame will reveal the presence of the lead. Such a mixture would render the hitherto resisting platinum absolutely worthless in the laboratory.