Page:Popular Science Monthly Volume 58.djvu/241

Rh lighters. The surface of the lake is 148 feet above the sea-level, and the pitch has flowed down to the sea from the lake in an immense stream that resembles a black glacier. Excavations made in this mass soon fill up again and all traces of them are in time obliterated, and buildings, the foundations of which are placed in or upon the pitch, are soon thrown out of perpendicular, from the unstable condition of the pitch, which appears to be moving or flowing towards the sea under a great pressure. These phenomena present the unique spectacle of a mass so solid as to be walked or driven over, and at the same time so plastic as to be in a state of unstable equilibrium, with constant ebullition from escape of gas and also in constant motion towards the sea.

Before the pitch is put to any use it is refined. In the operations attending its shipment and subsequent removal from the hold of the

ship, it has been very much broken up, and much of the gas has escaped with some of the water. In this condition it is put into enormous kettles, which are heated from above downward, and very slowly, until the contents of thirty tons or more are melted. The heat necessary to melt the pitch expels the water, the fragments of wood and other light impurities rise to the surface, and the heavy mineral matter, in large part, sinks to the bottom. The clean pitch between them is drawn off into barrels.

A more primitive method of refining the pitch is used at the island, where the pitch is boiled in old sugar kettles and skimmed, when the 'dean pitch is ladled into barrels and enters commerce as 'épurée.'

In the neighborhood of Trinidad, on the mainland of Venezuela, is another so-called Bermudez lake. It is found in a low savannah,