456 



same proportion as the silver. By steeping for a few minutes in 

 nitrous acid the silver is then dissolved ; but the gold or platina re- 

 main unaffected, and require merely to be washed in distilled water 

 in order to free them from any portion of the solution or other little 

 impurities that may adhere during the solution. 



The method employed by the author for coating gold wire is 

 attended with more difficulty than he expected. A rod of silver 

 having been previously drawn of considerable thickness, a hole was 

 drilled through it longitudinally, and into this hole a gold wire was 

 inserted so as to fill the hole. But in consequence of the toughness 

 of fine silver, the operation of drilling was found extremely difficult, 

 and this method was afterwards abandoned. It was found that 

 platina might be advantageously substituted for gold, as in that case 

 the first drawn wire might be coated with silver by fixing it in the 

 axis of a cylindrical mould, and then pouring melted silver to fill the 

 mould. The platina employed for this purpose was fused by the 

 flame of a spirit lamp impelled by a current of oxygen, as contrived 

 by Dr. Marcet: this platina having then been drawn alone to a wire 

 3-j-r of an inch in diameter, it received a coating of silver that was 

 just 80 times the thickness of the platina : accordingly when the 

 silver was reduced by drawing to -^^ of an inch in diameter, that 

 of the platina was - g o o o ' but nevertheless it remained surprisingly 

 tenacious in proportion to its substance. The greatest relative 

 tenacity is however thought to have been at about -nnhnr of an inch, 

 which supported 1-j- grain before it broke. Accordingly this wire 

 admitted being drawn considerably finer, and the author has even 

 obtained portions as slender as -g-o-ffTro- of an inch; but these were 

 only in very short pieces, being hi many places interrupted so that 

 he could place no reliance upon any trials of their tenacity. 



Some precautions are added respecting the method of freeing 

 these wires from their coating of silver, with the recommendation 

 of some little contrivances which the author has found convenient 

 in handling objects so liable to be injured. 



Description of a single-lens Micrometer. By William Hyde Wollaston, 

 M.D. Sec. R.S. Read February 25, 1813. {Phil. Trans. 1813, 

 p. 119.] 



The author, being unable to measure some of his very small wires 

 so accurately as he wished by any means at present in use, contrived 

 the method here described, which he recommends as fully answering 

 his expectations. 



A lens having its focus at one twelfth of an inch is mounted in a 

 plate of brass, and by the side of it is made a small perforation, as 

 near to its centre as ^-th of an inch. 



When a lens thus mounted is placed before the eye for the pur- 

 pose of examining any small object, the eye can at the same time see 

 distant objects through the adjacent perforation, by reason of the 

 magnitude of the pupU, which is sufficient for receiving rays through 



