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The great French crystallographer, Abbe Hauy, relates his experiments on a tourmaline crystal. He set this crystal in steel clamps, with a long stem which was inserted in a wooden handle, and then subjected the tourmaline to the heat of a brasier. As the heat augmented and penetrated the stone, its natural electric force became decomposed, the two component fluids being forced to separate from each other. It was now necessary to cool the tourmaline off a little; when too much heated the electrical phenomena were interrupted; they were also diminished in intensity when the stone became cool again. The perfect crystal chosen for experiment clearly showed the negative and positive electrical poles; even the smallest pieces showed this, and, indeed, if a very small piece were broken off the positively electric side of a crystal, it would preserve this positive electricity and soon develop a negative electricity also.
We may be somewhat loath to doubt the tale that little Dutch children were the first to note what to them was the queer action of some bits of tourmaline, but preference should probably be given to the statement that the discovery of the electric phenomena induced by heating in these stones was due to the fact that some Dutch jewellers put specimens of tourmaline in the fire to test their hardness, and then found that the stones attracted or repelled the ashes of the fire.
Toward the middle of the eighteenth century Dr. Haberden, of London, confirmed the deductions of Lemery and the somewhat later experiments of the German physicist Aepinus, and the gay world of London took up the idea, causing the new stone to become a great favorite with the fashionable. One of Hogarth's inimitable designs depicts a spendthrift fop who has just been arrested while his attention was riveted on the strange phenomena shown by the tourmaline.
In view of the important experiments made by Benjamin Franklin in the then almost unexplored field of electricity, it is easy to understand that the accounts of the newly-discovered electric properties of the tourmaline should have possessed considerable interest for him. This is testified to by a letter he addressed to Dr. William Haberden, June 7, 1759. Herein he expresses his thanks for two tourmalines his correspondent had sent him, and states that he is returning the smaller one. Of the electric phenomena he writes that he had heard some "ingenious gentlemen abroad" had denied the negative electricity displayed by one side of a tourmaline, but he believes the failure to observe could be explained by defective cutting of the specimens used, the positive and negative planes having perhaps been obliquely placed; to obviate this he suggests that the positive and negative sides should be accurately determined before the operation of cutting begins. The larger of the specimens sent by Dr. Haberden was retained by Franklin, who had it mounted on a pivot in a ring, so that either side could be turned outward at will. He notes as a curious circumstance that when he wore this ring, the natural heat of the finger sufficed to charge the stone, causing it to attract light bodies. Several of his experiments were made with a cork ball suspended by a thread, and he claims that the attractive force of the positive face was increased by coating it with gold-leaf attached to the stone by white of egg. This greater effect he supposed "to be occasioned by the united force of the different parts of the face collected and acting together through the metal." |
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Precious Stones Guide Vol 8
>> About the Phenomena of the Great Tourmaline Gem Stone
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