That copper should not have been employed, long before brass and other metals, in serving mainly for lightning conductors, its pre-eminence for this purpose being undisputed, would seem a strange fact, were it not explicable on several grounds. The first was the cost of the metal, which, though varying in price, is seldom less than six or seven times that of iron. It was needless for the advocates of copper as conductors--and there were not a few from the time its high conductive power had been demonstrated--to say that if copper was six times as dear as iron, it was likewise six times better as a carrier of the electric force, and that consequently the price, in respect of applicability for lightning protection, was in reality the same. But the reply to this was that copper, being one of the most expensive metals, except the so-called ‘precious’ ones, was exposed to the temptation of theft, and ought therefore not to be employed, since it was possible that vagrants, or other people, might tear off at any time the, in more than one sense, valuable pieces of metal protecting buildings against destruction from lightning. The argument, perhaps, was not worth much, but a better one not mentioned was in the background. It was, till quite recent times, an achievement of the greatest difficulty to manufacture long rods or bands of sufficiently pure copper to serve as lightning conductors. Sir William Snow Harris attempted, as already related, to get over this impediment by taking short plates, and fastening them together, and over each other, by copper nails. But this process, besides being enormously expensive, was in many other respects unsatisfactory, notably in that it made a shifting of the plates possible, and by the destruction of a few of them ruined the whole system. The pith and substance of the whole was the technical difficulty of hammering or drawing pure copper out into great lengths. That it must be pure was essential, the fact being thoroughly established that the electric conductivity of copper, mixed with impurities such as arsenic, is often not two-thirds, and sometimes not as much as one-half, that of the pure metal. This was conclusively shown by Sir William Thomson in a series of researches, and likewise by that distinguished investigator in the conductivity of metals, Professor Matthiessen. The latter, while placing copper on the same rank with silver, and far above gold--100 to 78--furnished the following instructive list as to the relative value of different kinds of copper:— Pure copper 100·00 Best American copper 92·57 Australian copper 88·86 Russian copper 59·34 Spanish ‘Rio Tinto’ copper 14·24