Page:Proceedings of the Royal Society of London Vol 1.djvu/476

Rh 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 unaﬁ'ected, 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 i attended with more difﬁculty than he expected. A rod of silver having been previously drawn of considerable thickness, 8. hole was drilled through it longitudinally, and into this hole a gold wire was inserted o 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 awire 1+,- 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 1+, of an inch in diameter, that of the platina was 714,11; but nevertheless it remained surprisingly tenacious in proportion to its substance. The greatest relative tenacity is however thought to have been at about W of an inch, which supported 1x5- g'rain before it broke. Accordingly this wire admitted being drawn considerably ﬁner, and the author has even obtained portions as slender as “.1,” of an inch; but these were only in very short pieces, being in 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.

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 1ﬁrth of an inch.

When a lens thus mounted is placed before the eye for the purpose 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 pupil, which is suﬂicient for receiving rays through