John Percy to Faraday   23 October 1844

Birmingham, Oct 23d 1844.

My dear Sir

Be pleased to accept my best thanks for your valuable present which I received last evening¹[.] You will much oblige me by accepting the specimen of nickel and cobalt. I value a specimen only in so far as it can be rendered subservient to science; and, therefore, have great pleasure in transferring the specimens in question to you. If you require specimens of manganese and tungsten, I think I shall be able to furnish you with them in a short time. I had heard from Mrs. Faraday of your expedition to the North², and I am now glad to hear that that expedition will be attended with benefit, it may be, to thousands of our poor miners, of whom we have so many in the vicinity of Birmingham. It occurred to me when talking over the subject with Mr. Solly³ one morning recently, that the question resolved itself into two particulars; first, the prevention, by proper ventilation, of the accumulation of the explosive gas; and, secondly, the detection and destruction of it in the event of its accidental accumulation. And it struck me, - I may have stumbled upon a mare's nest, - that the second object might possibly be accomplished by carrying into the various recesses of the mine a wire, intercepted at proper intervals, wherever accumulation of the gas in question would be likely to occur; so that by connecting this wire with a powerful hydro-electric machine the mine might, in the absence of the miners, be, from time to time, tested in respect to the presence of such explosive mixture. I suppose that by a proper arrangement of this kind, the electric spark might be made to traverse the mine, and to explode any mixture of carburetted hydrogen and air where the former had accumulated to a sufficient extent. One machine might be made available for many contiguous or adjacent mines. Probably, the notion may have long ago occurred to many persons. You will pardon me, I hope, for thus venturing to obtrude a notion, which may appear to you crude and impracticable, upon your attention.

I do assure you that Mrs. Percy⁴ and myself will be delighted again to entertain Mrs. Faraday and yourself. Your visit was a source of great delight to us both. And I do indulge the hope that in a short time you will again favour us with your company. You shall do just as you like in every respect. There yet remain many interesting manufactories to entice you.

I now believe that the notion concerning the transparency of the ultramarine particles originated in an optical illusion. However, I hope to be able to work out the subject satisfactorily. My friend, Mr. Shaw⁵ and I have entered upon a long investigation of the analysis of photographic phaenomena⁶; and I trust we shall arrive at some correct results in respect to the chemistry of these phaenomena.

Mr. Marrian⁷ whom you saw here, much regretted that he did not mention to you a curious phaenomenon which he observed a long time ago concerning the production of sound by the electric current. His experiment is this. Through the axis of a coil of covered wire place a long bar of iron, say six or eight feet long, and let it rest at each end on clay. Then, by passing a current through the coil at the instant of contact and breaking of contact a sound is produced similar to that produced by striking the bar on its end. He has varied the experiment in a great variety of ways and has apparently avoided every source of fallacy. No sound is produced when a bar of copper is similarly treated. He has, at the suggestion of Forbes, and others, just sent a paper on the subject to the Philosophical Magazine⁸.

I must, indeed, apologise for troubling you with so long a letter, and so taking up so much of your valuable time.

With best compliments to Mrs. Faraday and yourself, in which Mrs. Percy desires cordially to join.

Believe me, my dear sir, | With great respect | Yours very truly | John Percy

Dr. Faraday