For the purpose of removing the coating of silver which surrounds it, the wire must be steeped for a few minutes in warm nitrous acid, which dissolves the silver without danger of doing any injury to the gold. And though it might be difficult in this manner to preserve any considerable length of this wire, it is of little importance for any of those uses to which it is likely to be employed.
In my endeavours to make slender gold wire by the method above described, the difficulty of drilling the central hole in a metal so tough as fine silver, was greater than I had expected, and I was induced to try whether platina might not be substituted for the gold, as in that case its infusibility would allow me to coat it with silver without the necessity of drilling.
Having formed a cylindrical mould :1 of an inch in diameter, I fixed in the centre of it a platina wire previously drawn to the ,h of an inch, and then filled the mould with silver. When this rod was drawn to my platina was reduced to and and by successive reduction I obtained wires of and each excellent for applying to the eye-pieces of astronomical instruments, and perhaps as fine as can be useful for such purposes.
Since this has been the primary object that I had in view, I should have thought my time ill employed in pursuing farther the practical application of a me thod to which there seems no limit, except the imper fections of the metal employed. But as I found by trial the tenacity of these wires to be greater than was to be expected in proportion to their substance, that circumstance excited some doubts regarding the correctness of the estimate by which their diameter had been deduced. Other wires were consequently drawn with the utmost care, as to the quality and substance of the platina employed, and as to the proportional reduction of its diameter in the process of wire-drawing.
The extremity of a platina wire having been fused into a globule nearly nth of an inch in diameter, was next hammered out into a square rod, and then drawn again into a wire of an inch in diameter. One inch of this wire duly coated with silver was drawn till its length was extended to 182 inches, consequently the proportional diminution of the diameter of the platina will be expressed by the square root of 182, so that its measure had become . The specific gravity of the coat of 253X 13,5 3425 wire was assumed to be 10,5, and since the weight of 100 inches was 114 grains, its diameter was in 1 — ferred tobe of an inch, or just eighty times that 42,8 of the platina contained.
With portions of the platina wire thus obtained, and successively reduced in diameter, I had an op portunity of repeating the trials of its tenacity with greater confidence in the justness of the estimate, and the results showed generally (though with some accidental exceptions), that the process of wire drawing, which is well known to improve the strength of metals within moderate limits, continued also to add something to the tenacity of platina even as far as 18,000 — of an inch, which supported 11 grain before it broke; but the wire on which these experiments were made began to be impaired by repetition of the operation; so that, although I afterwards obtained portions of it as small as — 30,000 of an inch in diameter, it was in many places in terrupted, and I could place no reliance upon any trials of its tenacity.
There are some little circumstances in the man agement of these fine wires, which it may be of advantage to describe for the assistance of those who would apply them to any useful purpose. When the diameter is not less than aro- or saga of an inch, the difficulty of seeing and applying them in short pieces is not considerable; but when their diameter is farther reduced, and their length as much as an inch or more, the slightest current of air is sufficient to defeat all attempts to lay bold of an object so difficult to sec, and so impossible to feel; it is therefore necessary to retain a part of the silver coating at each extremity, which, at the same time that it assists in finding the end, also serves to stretch the wire with a certain moderate tension, and affords the means of attaching it in any required position.
The method which I have found most convenient is to bend a portion of the coated wire into the shape of the letter U, with small hooks at its upper extremities. In this form it will conveniently hang upon a wire of gold or of platina, with the lowest part immersed in nitrous acid, till the coating of silver is removed from that part. It may then, without difficulty, be lifted from its place, by one of the hooks alone, to any other situation, or sus pended by it, with the other hook downwards, as the means of attaching a small chain, or other series of equal weights in trials of its tenacity."