in the natural history of the ancients, the name of the stone used by way of touchstone for the trial of gold and silver, and called by some Heraclius lapis. Both of these names were also applied by the ancients to the loadstone; and hence has arisen no small misunderstanding of their works. Pliny has observed, that both the loadstone and touchstone were at times called Lydius and Heraclius lapis.

The true lapis Lydius, or touchstone, was anciently found only in the river Tmolus; but it was afterwards discovered in other places, and is now very common in many of the German rivers. The ancients give us very remarkable and circumstantial accounts of the uses they made of it; and it is plain that, by means of it, they were able to discern the alloys of gold with very great exactness. We at present use several different stones under this name, and for the same purpose. In Italy, a green marble called verdello is most frequently employed; and with us small pieces of basalt.