COPPER ON THE POTOMAC.– It is stated that the Potomac Copper Company is working some valuable mines on the immediate neighborhood of the Potomac river. At thirty feet it obtains ore yielding 30 per cent of the metal, which is far greater than the yield of ore from the English mines. It can send the metal at the low cost of $1 50 per ton to Baltimore market. The capital of the company is $1,000,000 in 100,000 shares of $10 each.