Page:Proceedings of the Royal Society of London Vol 1.djvu/127

Rh and pressure. In the second, 200 pieces of gold differently alloyed were inclosed within a wooden box, which was kept constantly turning i'ound, until, by the continued rubbing and striking of the pieces against each other, and against the sides of the box, they were found to be perceptibly diminished. And in the third mode, the pieces to be examined were pressed against the rim of a ﬂat horizontal wheel, by means of equal weights, so that by turning the wheel round, they all suffered an equal degree of friction. The part of the wheel against which the pieces rubbed was sprinkled or coated with some kind of powder, which was varied in the different experiments.

The general results of the many experiments made with this appa- ratus were, 1. That when equal friction, assisted by a moderate pres~ sure, takes place between pieces of coin which are in cash series of a similar quality, then, abrasion is most commonly produced in an inverse ratio to the degree of ductility ;——2. That the contrary effect happens when pieces of different qualities rub against each other, the more ductile metal being then worn by that which is harder ;— and 3. That earthy powders and metallic ﬁlings produce similar ef- fects, and tend to wear the different kinds of gold in proportion to their respective degrees of ductility.

The practical inferences to be deduced from these results are, that pure gold, being extremely ductile, is not the most proper to be formed into coin; that gold, on the other hand, brought by its alloy. to the greatest degree of hardness that will bear the manipulation of coining, will be so destructive to the instruments in the Mint, as to render the expense occasioned by this detriment much greater than the small saving that would accrue from the greater durability of the metal; and that hence gold of a moderate ductility must be that .which is best adapted for coin, which degree of ductility will he found in the standard proportion'of one-twelfth of alloy consisting of about equal parts of silver and copper.

Several incidental circumstances are mentioned at the close of this paper, some of which relate to the cause of the changes of colour in gold coins, which are ascribed to certain chemical changes in the alloy near the surface of the piece during the processes of annealing and blanching. We are also informed that the obliteration of the impressions on gold coins is not always attended with a diminution of weight, but that the supposed abrasion of the prominent parts is in fact a depression of those parts into the mass, bringing them to a level with the rest.

Upon the whole, our author concludes that the great loss which the gold coin of this kingdom is stated to have sustained, cannot possibly be attributed to any important defect in the composition or quality of the standard gold; and that all that can be said upon this subject is, that some portion of this loss may have been caused by the rough impression and milled edge now in use, by which each piece of coin acts and is acted upon by the others, in the manner of a ﬁle or rasp.