There is but one more point suggested by these mines, which it is not well to pass over when we are considering the working of them in ancient times. Nothing is more poisonous than the smoke from lead-mines; and for this reason the people at Ergasteria have built a chimney more than a mile long to the top of a neighboring hill, where the smoke escapes. Even so, when the wind blows back the smoke, all the vegetation about the village is at once blighted, and there is no greater difficulty than to keep a garden within two or three miles of this chimney. As the Athenians did not take such precautions, we are not surprised to hear from them frequent notices of the unhealthiness of the district, for when there were many furnaces, and the smoke was not drawn away by high chimneys, we can hardly conceive life to have been tolerable. What then must have been the condition of the gangs of slaves which Nicias and other respectable and pious Athenians kept in these mines? Two or three allusions give us a hideous insight into this great social sore, which has not been laid bare, because the wild district of Laurium, and the deep mines under its surface, have concealed the facts from the ordinary observer. Nicias, we are told, let out one thousand slaves to Sosias the Thracian, at an obolus a day each—the lessee being bound to restore them to him the same _in number_. The meaning of this frightful contract is only too plain. The yearly rent paid for each slave was about half the full price paid for him in the market. It follows that, if the slave lived for three years, Nicias made a profit of 50 per cent. on his outlay. No doubt, some part of this extraordinary bargain must be explained by the great profits which an experienced miner could make—a fact supported by the tract on the Revenues, which cannot date more than a generation later than the bargain of Nicias. The lessee, too, was under the additional risk of the slaves escaping in time of war, when a hostile army might make a special invasion into the mountain district for the purpose of inflicting a blow on this important part of Athenian revenue. In such cases, it may be presumed that desperate attempts were made by the slaves to escape, for although the Athenian slaves generally were the best treated in Greece, and had many holidays, it was very different with the gangs employed by the Thracian taskmaster. We are told that they had three hundred and sixty working days in the year. This, together with the poison of the atmosphere, tells its tale plainly enough.